Two-stroke cycle engine

ABSTRACT

This invention concerns a two-stroke cycle engine, more specially it concerns a two-stroke cycle engine using a preceding air-layer for scavenging. It has a scavenger passage connected to a branching scavenger passage opened to said scavenging port. The engine has a connecting passage to link the air passage and the fuel passages so that negative pressure in the air passage forces the fuel-air mixture in the fuel passage into said air passage. Further, the engine according to this invention has a removable guide with a surface forming a curved smooth channel which is attachable to the scavenger passage in the crankcase from the mounting surface, and forms a portion of said scavenger passage with the curved channel. The blow-up angle of the scavenger passage varies along the circumferential direction of the cylinder. The crankcase is configured in such a way that the front and rear portions, which are separated by a block, and a scavenger passage is provided inside both said front and rear portions of said crankcase, and the cylinder. The air cleaner has two air passages running from it in parallel, the first one is connected to said air passage, and the second one is connected to the air inlet of the carburetor to provide air for the fuel passage, and a choke valve on the air cleaner is provided to open and close both of the first and second air passages.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention concerns a two-stroke cycle engine which uses alayer of scavenging air pressurizing the crankcase. More specifically,it concerns a small two-stroke cycle engine using a preceding air-layerfor scavenging, which drives a layer of scavenging air in advance of thefuel-air mixture.

[0003] 2. Description of the Related Art

[0004] Two-stroke cycle engines belonging to the prior art takeadvantage of the fact that a negative pressure is created in thecrankcase when the piston reaches the top of its stroke. This negativepressure causes the fuel-air mixture to be sucked into the crankcase.When the piston reaches the bottom of its stroke, the pressurizedfuel-air mixture in the crankcase reaches the scavenging port and isconducted from the crankcase into the combustion chamber. The fuel-airmixture fills the combustion chamber, pushing the exhaust gases ahead ofit. In this scavenging process, the opening duration of the scavengingport and the exhaust port experiences significant overlap, with theresult that approximately 30% of the fuel-air mixture is sucked out withthe exhaust gases. This is the primary cause of the large component ofTHC (total hydrocarbons) in the exhaust, and it results in the wastageof fuel.

[0005] To reduce the quantity of fuel-air mixture which is pushed out ofthe combustion chamber, scavenging air designs which drive a layer ofair ahead of the fuel-air mixture have been proposed. In engines whichuse scavenging air, the fuel-air mixture goes into the crankcase as thepiston travels upward in the intake process. At the same time, air issucked into the crankcase through a scavenger passage connected to thescavenging port so that the passage is filled with air. In thecombustion and exhaust processes which occur when the piston drops andthe scavenging port is open, the air in the scavenger passage is forcedinto the combustion chamber ahead of the fuel-air mixture to scavengethe exhaust gases from the combustion. Immediately after the scavengingair, the fuel-air mixture is admitted into the combustion chamber. Thisscavenging-air method reduces the quantity of fuel-air mixture which ispushed out of the combustion chamber to one third that which occurredwith prior art engines.

[0006] A design for a scavenging-air two-stroke cycle engine whichforces a layer of air ahead of the fuel-air mixture, in which the fueland air valves on the carburetor are realized as a single valve, isdisclosed in the Japanese Patent Publication (Kokai) 10-252565.

[0007] In the prior art scavenging-air engine which drives a layer ofair ahead of the fuel-air mixture, the preceding layer of air admittedto the cylinder and crankcase through the air control valve was routedthrough the same number of passages (either two or three) as there werescavenging ports downstream from the air control valve. These wereconnected to the passages for the scavenging ports of the cylinders byrubber tubes. The air was fed through lead valves on the scavengerpassages to passages on the cylinder and crankcase.

[0008] The air introduced via the air control valve was sucked into thecrankcase temporarily when the cylinder of the piston was pressurized.When the piston dropped and scavenging occurred, the scavenging air wasled into the combustion chamber from the scavenging port.

[0009] In another prior art design proposed in the Japanese PatentPublication (Kokai) 7-139358, an air passage was provided which fed intothe scavenger passage at a location adjacent to the scavenging port. Anon-return valve was provided on the air passage, as was a controlvalve. The control valve was interlinked with the operation of theengine throttle. In this engine, the crankcase experienced negativepressure when the piston was up. At the same time that the fuel-airmixture was sucked into the crankcase through its supply port, thenon-return valve was opened and the air was sucked in through the airpassage. This air would completely or partially fill the scavengerpassage. When the piston fell during the ignition and exhaust processesand the scavenging port was opened, first the air would rush into thecombustion chamber and then the fuel-air mixture would be supplied.

[0010] With this prior art technique, a means was devised which wouldsupply the air from the scavenging port to scavenge the combustionchamber quickly at the start of the scavenging process so as to minimizethe quantity of fuel-air mixture lost through the exhaust port. Thisdevice admitted the fuel-air mixture from the crankcase into thecombustion chamber via the scavenging port with a slight delay after thescavenging air was admitted.

[0011] This sort of two-stroke cycle engine which admitted a layer ofair in front of the fuel-air mixture reduced the quantity of mixtureexhausted with the combustion gases, prevented an excessive quantity ofTHC (total hydrocarbons) from being exhausted, and minimized thequantity of fuel wasted.

[0012] In the preceding air-layer type two-stroke cycle engine proposedin the Japanese Patent Publication (Kokai) 10-252565, the preceding airwas brought in through a number of rubber tubes with lead valves whichwas equal to the number of scavenging ports. The design thus required alarge number of parts and assembly processes, both of which drove thecost up. Furthermore, the supply passages for the air were provided onthe outside of the cylinder, so the dimensions of the engine in itsaxial direction were increased.

[0013] In a two-stroke cycle engine, combustion must be kept stable bysupplying a rich mixture with little air when the engine is operatingunder a light load, including when it is idling, and a comparativelythin mixture when it is operating under a heavy load. This will reducefuel consumption and decrease the harmful component of the exhaust gas.However, in the prior art design proposed in the Japanese PatentPublication (Kokai) 7-139358, the airflow supplied via the supplypassages during scavenging is not controlled to conform to the operatingstate of the engine.

[0014] Under light load conditions, then, such as when the engine isidling, too much air is supplied; and it would be difficult to stabilizecombustion by limiting the quantity of air admitted to produce a richmixture. Similarly, it would be difficult to maintain a thin mixtureunder heavy load conditions in order to reduce the pollutants in theexhaust gas and lower the fuel consumption.

[0015] In the invention disclosed in the Japanese Patent Publication(Kokai) 9-125966, a mixture control valve is provided to open and closethe mixture passage which connects the carburetor to the crankcase, andan air control valve is provided to open and close the air passage whichconnects the air cleaner. The mixture control valve and air controlvalve are linked so that it is possible to control the flow rate of thefuel-air mixture and that of the air in such a way that their ratioremains constant.

[0016] In this type of preceding air-layer type two-stroke cycle engine,when the engine is idling the negative pressure in the air passageincreases until it is higher than that in the fuel mixture passage. Thiscauses the throttle to open more, suddenly increasing the speed of theengine. The delay in the fuel supply allows the excessively rich fuelmixture to be thinned out by radically increasing the quantity ofpreceding air. The extra air reduces the concentration of the fuelmixture.

[0017] But when the engine is operating at high speed, an increase inthe quantity of air will not be followed by an increased quantity offuel. The concentration of the fuel will decrease and proper combustionwill no longer be possible. Problems with acceleration or engine cut-offmay result.

[0018] However, in the inventions disclosed in the Japanese PatentPublications 7-139358, 10-252565 and 9-125966, no means are provided tocontrol the ratio of air flow to air-fuel mixture flow during normaloperation so as to prevent an excessive quantity of air from beingsupplied when the engine suddenly accelerates as was described above.

[0019] Furthermore, when this sort of two-stroke cycle engine is used ina lawnmower, in many cases it must operate while mounted obliquely. Whenan obliquely mounted engine operates, fuel collects in the portion belowthe passage for the fuel-air mixture. When the position of the enginechanges, this fuel is sucked in suddenly, resulting in combustionproblems due to excess fuel in the engine.

[0020] However, the prior art techniques did not offer any method tocounteract problems arising from the engine being operated while mountedobliquely.

[0021] Generally in two-stroke cycle engines, the passage for thescavenging air which leads into the chamber inside the crankcase followsa smooth curve inside the crankcase, goes through the surface where thecrankcase is attached to the cylinder, and is connected to thescavenging port in the cylinder.

[0022]FIG. 39 shows an example of a scavenger passage in a two-strokecycle engine belonging to the prior art. In this drawing, 1 is theengine, which is configured as follows.

[0023]2 is the cylinder; 5 is the crankcase. The cylinder 2 andcrankcase 5 are fixed to each other with gasket 3 between them by bolts110 at surfaces 04 and 05. 6 is the crankshaft, 7 is the cylinder head,10 is the air passage and 60 is the center of the crankshaft.

[0024]9 is the scavenging port, which opens into the side of thecylinder 2. 109 a is the scavenger passage formed in the cylinder 2,which connects to the scavenging port 9. 109 c is the inlet for thescavenger passage formed in the crankcase 5, which opens into the crankchamber. 109 b is the scavenger passage in the crankcase 5. It follows asmooth curve inside the crankcase 5 and connects scavenger passage 109 ain the cylinder 2 with scavenging inlet 109 c.

[0025] In this two-stroke cycle engine, the scavenger passage comprisespassage 109 b in crankcase 5 and 109 a in cylinder 2, which meet at thesurfaces 04 and 05. Because scavenger passage 109 b in crankcase 5 iscurved, it has a portion 16 which protrudes between the upper wall ofthe passage 109 b and surface 04.

[0026] In the Japanese Patent Publications 58-5423 and 58-5424,scavenger passages are disclosed such that a curved scavenger passage inthe crankcase and a scavenger passage in the cylinder come together atthe surface where the crankcase and the cylinder are fixed to eachother.

[0027] However, in the configuration of the scavenger passage in thetwo-stroke cycle engine shown in FIG. 39, scavenger passage 109 b incrankcase 5 is a curved passage with a portion 16 which protrudesbetween the upper wall of the passage 109 b and surface 04. When thecrankcase is cast, the die to form scavenger passage 109 b in crankcase5 cannot be removed as a single die in the direction of axis 61 of thecylinder.

[0028] With the prior art design shown in FIG. 39, then, to enable thedie for scavenger passage 109 b to be removed, several different dieswere combined to cast the scavenger passage. This complicated thecasting work and increased the number of casting processes. Also,because it required combining a number of dies, the probability of adefective cast due to slippage of a die increased.

[0029] In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424,too, a protruding portion is formed between the upper wall of thescavenger passage and the surface where the cylinder is fixed to thecrankcase. These designs, then, suffer from the same problems as werejust described.

[0030] Further, with the prior art design shown in FIG. 39, the curvedscavenger passage in the crankcase and the scavenger passage in thecylinder meet to form a scavenger passage which goes through the surfacewhere the crankcase and the cylinder are fixed to each other. The fuelin the fuel-air mixture which flows through the scavenger passage willthus seep into the microscopic gap between surfaces 04 and 05 wheregasket 3 is inserted. When the engine is operated in an obliqueposition, this fuel will return to the scavenger passage and result indefective combustion.

[0031] In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424,too, a scavenger passage in the crankcase and a scavenger passage in thecylinder meet to form a common passage which goes through the surfacewhere the crankcase and the cylinder are fixed together. These designsthus suffer from the same problem which is described above.

[0032] In a two-stroke cycle engine with a pressurized crankcase, thedesign takes advantage of the fact that a negative pressure is createdin the crankcase when the piston reaches the top of its stroke. Thefuel-air mixture is sucked into the crankcase through the air inlet.When the piston reaches the bottom of its stroke, the scavenging portopens and the pressurized fuel-air mixture in the crankcase is conductedfrom the crankcase into the combustion chamber via the scavenger passageand the scavenging port. The fuel-air mixture fills the combustionchamber, pushing the exhaust gases ahead of it.

[0033] In the scavenging process in a two-stroke cycle engine, theopening duration of the scavenging port and the exhaust port experiencessignificant overlap. To address this problem, a number of devices havebeen proposed to prevent the fuel-air mixture from being sucked out withthe combustion gases and to insure that the mixture fills the combustionchamber uniformly.

[0034] One such device is proposed in the Japanese Utility ModelPublication (Kokai) 1-44740. In this proposal, a two-stroke cycle enginehas two scavenger passages, one on the right and one on the left, whichlead up from the crankcase. Their upper ends curve toward the axialdirection of the cylinder, and they lead into the cylinder. The anglesat which the surfaces of the upper walls of the curving scavengerpassages meet the cylinder vary continuously from one side to the other.

[0035] In this sort of scavenging two-stroke cycle engine with apressurized crankcase, it is necessary to reduce the quantity offuel-air mixture which escapes with the exhaust gases, eliminate theexhaust of a large quantity of THC (total hydrocarbons) and minimize thewastage of fuel.

[0036] In the scavenging two-stroke cycle engine with a pressurizedcrankcase proposed in the Japanese Utility Model Publication (Kokai)1-44740, the angles at which the surfaces of the upper walls of the topsof the scavenger passages meet the cylinder, that is, the angles atwhich the air is blown into the cylinder, do vary continuously from oneside of the scavenging port to the other. However, as can be seen inFIG. 3 of the same publication, the angles θ of the airflow differ fromeach other. The portion (θa) nearer the exhaust port, which is shown inFIG. 3 (a) is larger than the portion (θc) nearer the intake port, whichis shown in FIG. 3 (c).

[0037] Thus in the prior art design proposed in the Japanese UtilityModel Publication (Kokai) 1-44740, the angle θ of the airflow in thelocation closer to the exhaust port is made larger. As a result, afterthe fuel-air mixture forced into the combustion chamber from the portionof the scavenger passage closer to the exhaust port reaches the top ofthe combustion chamber, it is liable to be caught in the flow ofcombustion gases travelling toward the exhaust port. The fuel-airmixture supplied from the location closer to the exhaust port, then, islikely to escape out the exhaust port with the combustion gases whichcomprise the exhaust gas. This will increase the quantity of the THC(total hydrocarbons) which are exhausted and the quantity of fuel whichis wasted. The scavenging efficiency will decrease, the density of thefuel-air mixture filling the combustion chamber will be lower, and theengine output will go down.

[0038] In a two-stroke cycle engine which is used in a lawnmower, thelong scavenger passage which connects the scavenging port to thecrankcase and supplies the air-fuel mixture from the crankcase to thecombustion chamber must be formed in both the crankcase and thecylinder. The crankcase and cylinder, which are generally made of castaluminum, must assume a complicated shape, so that their castingrequires many processes.

[0039] Two-stroke cycle engines for universal applications have beenproposed in the Japanese Patent Publication (Kokai) 58-5424 and theJapanese Utility Model Publication (Kokai) 4-26657, among others.

[0040] In the Japanese Patent Publication (Kokai) 58-5424, the crankcasehas both a primary and an auxiliary scavenging port. The two scavengerpassages which lead into the interior of the crankcase, i.e., into thecrank chamber, go from the interior of the crankcase through the surfacewhere the crankcase and the cylinder are fixed together. In thecylinder, these two scavenger passages connect to the primary andauxiliary scavenging ports.

[0041] In the Japanese Utility Model Publication (Kokai) 4-26657, twopairs of curved scavenger passages go from the interior of the crankcasethrough the surface where the crankcase and the cylinder are fixedtogether and then through the interior of the cylinder.

[0042] As has been discussed, this is an air-layer type scavengingtwo-stroke cycle engine. In it, a long scavenger passage which connectsthe scavenging port and the crankcase is formed in the interior portionsof the crankcase and the cylinder. In addition, an air passage is formedin the cylinder to transport the preceding air to the scavenging port.This air passage connects to some intermediate location on the scavengerpassage. The crankcase and cylinder, which are generally made of castaluminum, must be shaped in such a way that the scavenger passage formsa smooth channel in order to minimize the resistance experienced by thefuel-air mixture and the airflow. The shapes of the dies for the castingmust be simple, the number of dies must be small, and the engine must beable to be produced in a small number of production processes.

[0043] In the design in the Japanese Patent Publication (Kokai) 58-5424,however, the inventors limited their improvement to giving the two longprimary and auxiliary scavenger passages in the crankcase and cylinder asmooth contour and so reducing the resistance of the two channels. Theydid not devote any attention to improving the shape or number of diesused when the crankcase and cylinder were cast or to improving thecasting work by reducing the number of casting processes required.

[0044] In the design proposed in the Japanese Utility Model Publication(Kokai) 4-26657, the inventors limited their improvement to providing ashape for a curved scavenger passage running from the interior of thecrankcase through the interior of the cylinder and preventing thefuel-air mixture from escaping toward the exhaust side. In this priorart design, then, just as with that discussed above, no considerationwas given to the casting of the crankcase and cylinder.

[0045] In a two-stroke cycle scavenging engine using a layer of air,there is a scavenger passage which connects the scavenging port on theside of the cylinder and the crankcase; an air passage which connects tothe scavenger passage at a point midway along its length and suppliesscavenging air from the air cleaner to the scavenger passage; and apassage which supplies the fuel-air mixture produced in the carburetorto the crankcase. Before the fuel-air mixture is supplied from thescavenging port to the combustion chamber, a mass of preceding airfiltered by the air cleaner is conducted into the combustion chamber byway of the air passage, scavenger passage and scavenging port. This airscavenges the chamber, enhancing both the scavenging and the combustionefficiency.

[0046] Two inventions which have proposed such two-stroke cyclescavenging engines which use a layer of air are those disclosed in theJapanese Patent Publications (Kokai) 9-125966 and 10-252565.

[0047] In the invention disclosed in the Japanese Patent Publication(Kokai) 9-125966, there are two air cleaners. The outlet of one of theair cleaners connects via the carburetor to the supply passage for thefuel-air mixture. The outlet of the other air cleaner connects through acontrol valve to the passage which supplies the preceding air to thescavenger passage.

[0048] In the invention disclosed in the Japanese Patent Publication(Kokai) 10-252565, the passage which supplies the fuel-air mixture fromthe carburetor to the crankcase is parallel to the passage whichsupplies the preceding air to the scavenger passage. The air inlet ofthe carburetor and the inlet of the air supply passage are connecteddirectly to the outlet of the air cleaner.

[0049] When this air layer-type scavenging two-stroke cycle engine isstarted up, the quantity of air which is to go through the air supplypassage must be controlled and a negative pressure must be obtained. Thequantity of fuel-air mixture supplied to the combustion chamber from theair mixture supply passage by way of the crankcase and scavenger passagemust be increased to produce a rich mixture and so improve the startingcharacteristics of the engine.

[0050] In the invention proposed in the Japanese Patent Publication(Kokai) 9-125966, the air control valve is shut during start-up, so theair supply passage is closed. The air passage on the carburetor is open,and the fuel-air mixture supplied from the carburetor to the air mixturesupply passage is increased to produce a rich mixture. However, thisdesign requires two air cleaners, so the configuration is complicatedand large, which drives up the equipment cost.

[0051] In the invention proposed in the Japanese Patent Publication(Kokai) 10-252565, the air inlet of the carburetor and the inlet of theair supply passage are connected directly to the outlet of the aircleaner. For this reason it would be extremely difficult to completelyshut off the preceding air, that is, the air which flows into the airsupply passage, at the outlet stage of the air cleaner during start-up.

[0052] It is thus impossible, in this invention, to supply a richmixture to the combustion chamber. And since it is difficult to achievea high negative pressure as well, the start-up characteristics areunavoidably poor.

[0053] Furthermore, in the two prior art designs discussed above, thechoke valve which closes off the air passage when the engine is startedup is generally a rocking choke valve which rotates about its valveshaft. When the engine is connected to a lawnmower or other workingmachine and is to be operated, the operating lever of the choke valvegets in the way when the engine is started with the recoil starter.Also, the rocking diameter of the operating lever is considerable, whichmakes it difficult to operate and so affects the ease of operation.

SUMMARY OF THE INVENTION

[0054] The present invention was developed to address the problemsassociated with the prior art.

[0055] The first objective of this invention is to simplify theconfiguration of the scavenger passage in a scavenging two-stroke cycleengine using a layer of air, to make the engine smaller and lighter, toreduce the number of parts and processes required to produce the engine,and to reduce the production cost.

[0056] The second objective of this invention is to provide anair-layer-type scavenging two-stroke cycle engine such that the quantityof fuel-air mixture exhausted with the combustion gases is reduced, andat the same time the quantity of air supplied to the combustion chamberthrough the scavenging port is controlled via a cut-off valve so that itremains in proper proportion to the quantity of fuel-air mixture. Insuch an engine, an appropriate concentration of fuel-air mixture ismaintained through the entire operating range of the engine. Combustionremains constant when the engine is operated under a light load, and therate of fuel consumption and the proportion of pollutants in the exhaustare reduced when the engine is operated under a heavy load.

[0057] The third objective of this invention is, in an air-layer-typescavenging two-stroke cycle engine, to prevent the concentration of thefuel mixture from becoming too thin due to an excessive intake of airwhen the engine accelerates suddenly and so maintain proper combustion,thus preventing the engine from hesitating or cutting out.

[0058] The fourth objective of this invention is to prevent fuel fromcollecting at the bottom of the fuel passage when the engine isinstalled obliquely, and so prevent the defective combustion which wouldresult from an excess of fuel.

[0059] The fifth objective of this invention is to make it possible tocast the scavenger passage for a two-stroke cycle engine in such a waythat a single die could be used and the piece could be removed from thedie in the axial direction of the cylinder. The casting process would besimplified and the number of sub-processes reduced. This would preventdefects resulting from the dies slipping. It would also prevent theimperfect combustion which occurs when the engine is mounted obliquelydue to fuel flowing back into the scavenger passage from the surfacewhere the cylinder and crankcase are fixed together.

[0060] The sixth objective of this invention is, in a two-stroke cycleengine using pressurized air to scavenge the crankcase, to prevent thefuel-air mixture from escaping out the exhaust port, the reduce thequantity of THC (total hydrocarbons) in the exhaust, to improve theefficiency of the scavenging, to increase the concentration of thefuel-air mixture in the combustion chamber, and to improve thecombustion and so enhance the output of the engine.

[0061] The seventh objective of this invention is to provide atwo-stroke cycle engine with a scavenger passage in the crankcase andcylinder and a method of casting such an engine such that a smoothscavenger passage with little flow resistance could be formed,simply-shaped dies could be used to cast the engine, the number of diescould be reduced, and the number of casting processes could be reduced.

[0062] The eighth objective of this invention is to provide a two-strokecycle engine with a simple, compact, low-cost configuration which wouldbe able to supply a rich fuel-air mixture to the combustion chamberduring start-up, and which would enjoy improved start-up characteristicsas a result of achieving a high negative pressure.

[0063] The ninth objective of this invention is to provide a valve suchas a choke valve with a simple and compact configuration to improve theoperating characteristics of the engine.

[0064] This invention, then, comprises an exhaust port on the side ofthe cylinder; a scavenging port on the side of the cylinder; a fuelpassage, which supplies fuel-air mixture to the crank chamber throughthe intake port on the side of the cylinder during the time of theelevation of the piston; an air passage, which supplies scavenging airfrom the air cleaner towards the inner side of the engine; an insulator,in which the fuel passage and the air passage run in parallel; anon-return valve, which is provided on the insulator facing towards theinner side of the engine, to allow the scavenging air to flow onlytowards the inner side of the engine; a pair of branching air passagesto connect an air supply chamber provided at the inner side of thenon-return valve and a branching scavenger passage opened to thescavenging port, which are provided within the wall of the cylinder: anda pair of scavenger passages, one end of which is connected to thescavenging port, and another outlet end of which is opened to the crankchamber, the pair of scavenger passages are provided within the wall ofthe crankcase.

[0065] With this invention, scavenger passages are formed in halves,with one half of each inside the walls of the crankcase and the otherhalf inside the walls of the cylinder. This results in long scavengerpassages which, when filled with air, can scavenge the crankcase aheadof the fuel-air mixture. Since the mixture is supplied after thecrankcase is completely scavenged, the quantity of mixture lost throughthe exhaust port can be minimized.

[0066] Since all the scavenging and air passages are formed inside thecrankcase and cylinder, no external pipes or mounting hardware areneeded to create air passages. Both the parts count and the number ofassembly processes are therefore reduced.

[0067] Ideally, as in claim 2, in addition to the configurationdisclosed in claim 1, the end surface of the outlet end of the scavengerpassage in the crankcase forms right angles with respect to the axis ofthe crankshaft, and a microscopic gap is created between the end surfaceof the outlet and the end surfaces of the crank webs which areperpendicular to the crankshaft, which constitutes disk valves, as theopening area of the outlet of the scavenger passage varies as the crankwebs rotate.

[0068] As in claim 3, the opening area of the outlet of the scavengerpassage is formed so that the opening area opens more with the rotationof the crank webs as the opening area uncovered by the crank web growslarger.

[0069] If configured as described above, the area of the outlets of thescavenger passages will increase with the rotation of the crank webs. Inthis way the velocity of the scavenging air which flows from the outletsthrough the scavenger passages and from the scavenging ports into thecombustion chamber can be controlled. This allows us to reduce thequantity of fuel-air mixture which is dragged into the exhaust gasstream, and so minimize the loss of fuel-air mixture.

[0070] Ideally, as in claim 4, the branching air passages and thebranching scavenger passages formed on either side of the cylinder aresurrounded by virtually parallel walls which run in the same direction.

[0071] If configured in this way, the walls of the branching scavengingand air passages can be formed integrally to and virtually parallel withthe cylinder. This allows the cylinder to be cast using a single slidingdie. This simplifies the configuration of the die and reduces the costof producing it.

[0072] The invention disclosed in claim 5 of this application comprisesa two-stroke cycle engine using a preceding air-layer for scavengingwith an exhaust port on the side of the cylinder; a scavenging port onthe side of the cylinder; an intake port on the side of the cylinderfuel passage, which supplies fuel-air mixture through a mixture controlvalve on the carburetor to the crank chamber during the time of theelevation of the piston; a scavenger passage opened to the scavengingport; an air supply port, which supplies scavenging air from the aircleaner to the scavenger passage; a cam which is interlocked with themixture control valve; a cam follower which engages with the cam; and anair control valve in the upstream of an air passage which controls thediameter of the air passage, and the air control valve being operated bythe cam and the cam follower in such a way as to supply a quantity ofscavenging air proportional to the quantity of fuel-air mixturedetermined by the opening of the cam and the mixture control valve tocontrol the fuel-air mixture.

[0073] Ideally, as in claim 6, in addition to the configurationdisclosed in claim 5, the air control valve comprises a valve seatmidway along the air passage and an umbrella-type valve which can beattached to or removed from the valve seat and which opens and closesthe air passage, the cam is fixed to the rotary shaft of the mixturecontrol valve, the cam is configured with an inner cam which is formedon the inside of the edge at a given height raised up on the outer sidealong the circumference so that, if a spring exerts force in thedirection which closes the air control valve, when the edge of the innercam engages with the cam follower, the operation of the mixture controlvalve for the fuel-air mixture is transmitted to the air control valve,and the operation opens the air control valve against the force of thespring.

[0074] Ideally, like the means disclosed in claim 7, which is the secondpreferred embodiment of the internal cam in claim 6, the air controlvalve has, in addition to the configuration disclosed in claim 5, avalve seat midway along the air passage and an umbrella-type valve whichcan be attached to or removed from the valve seat and which opens andcloses the air passage, the cam is fixed to the rotary shaft of themixture control valve, the cam is configured with an inner cam which isformed on the inside of the edge at a given height dropped down on theouter side along the circumference so that, if a spring exerts force inthe direction which closes the air control valve, when the edge of theinner cam engages with the cam follower, the operation of the mixturecontrol valve for the fuel-air mixture is transmitted to the air controlvalve, and the operation opens the air control valve against the forceof the spring.

[0075] If configured in this way, the air control valve is interlockedwith the valve which controls the flow rate of the fuel-air mixture.Thus when the engine operates under a light load, the air control valveis closed more, and the quantity of air is reduced. This allows stablecombustion with a rich mixture. Under heavy load conditions, the aircontrol valve is opened more, and the engine operates with a thinmixture. In this way we can prevent noxious substances with high THC(total hydrocarbons) from being exhausted.

[0076] If configured as described above, the opening ratio of thethrottle valve and air control valve can easily be controlled inresponse to a change in the angular position of the mixture controlvalve.

[0077] Ideally, as in claim 8, in addition to the configurationdisclosed in claim 5, an air passage which connects to the outlet of thesupply passage for the fuel-air mixture runs inside the insulator. Thisinsulator is attached to the side of the cylinder downstream from theoutlet of the supply passage and the air control valve and runs in thesame direction as the air supply outlet. The air passage runs in thesame direction as the insulator.

[0078] The invention disclosed in claim 9 of this application comprisesa two-stroke cycle engine using a preceding air-layer for scavengingwith an exhaust port on the side of the cylinder; a scavenging port onthe side of the cylinder; a fuel passage, which supplies fuel-airmixture to the crank chamber through the intake port on the side of thecylinder during the time of the elevation of the piston; a scavengerpassage to be connected to the scavenging port; an air passage, whichsupplies scavenging air from the air cleaner toward the inner side ofthe engine; an insulator, in which the fuel passage and the air passagerun in parallel; a non-return valve, which is provided on the insulatorfacing toward the inner side of the engine, to open or close the airpassage by means of the negative pressure in the scavenger passage; anda connecting passage with a small diameter to link the air passage andthe fuel passages so that negative pressure in the air passage forcesthe fuel-air mixture in the fuel passage into the air passage.

[0079] With the invention disclosed in claim 9 of this application, thefuel-air mixture in the fuel passage is supplied to the air passagethrough a small-diameter connecting passage when the negative pressurein the air passage becomes greater than that in the fuel passage becausethe engine is idling. Thus when there is an excessive quantity of air inthe air passage during sudden acceleration, fuel-air mixture from thefuel passage will be introduced and mixed with that air.

[0080] By forcing fuel-air mixture into the airflow in the air passage,we prevent the new air supplied to the cylinder from the scavenging portfrom creating a mix with too much air. This method prevents the fuelmixture from becoming too thin during sudden acceleration of the engineand so improves the acceleration characteristics.

[0081] Furthermore, since the throttle opens during high-speedoperation, the pressure differential between the fuel and air passagesvirtually disappears. Thus virtually no fuel-air mixture will flow fromthe fuel passage through the small-diameter connecting passage and intothe air passage at high speeds. The method prevents fuel-air mixturefrom getting into the preceding air and so helps maintain the requiredexhaust characteristics.

[0082] When the engine is mounted obliquely, the fuel-air mixture in thefuel passage can flow through the small-diameter connecting passage intothe air passage. It will thus not collect in the bottom of the fuelpassage, so that a large quantity of fuel is suddenly sucked into thecylinder when the engine's orientation is changed. This design, then,will prevent imperfect combustion.

[0083] With the invention disclosed in claim 9, then, we can achieve anair-layer-type scavenging two-stroke cycle engine which produces theeffect through a very simple device, i.e., a small-diameter connectingpassage between the air and fuel passages which causes the negativepressure of the air passage to draw the fuel-air mixture in the fuelpassage into the air passage. This obviates the need for a complicatedcontrol device.

[0084] In invention disclosed in claim 10, the small-diameter connectingpassage in the configuration discussed in claim 9 connects the air andfuel passages at a point downstream from the non-return valve.

[0085] If configured in this way, the connecting passage goes into theair passage downstream from the non-return valve. When the engine ismounted obliquely, when it is, say, rotated 180°, the fuel whichcollects in the lower end of the fuel passage will flow through theconnecting passage into the air passage. This will prevent too much fuelfrom flowing into the combustion chamber and affecting the rate ofcombustion, improving both the engine's operation and its effect.

[0086] In invention disclosed in claim 11, the small-diameter connectingpassage in the configuration discussed in claim 9 can connect the airand fuel passages at a point upstream from the non-return valve.

[0087] If configured in this way, the connecting passage goes into theair passage upstream from the non-return valve. The diameter of themouth of the connecting passage can be increased without producing adrop in engine output. This further improves the accelerationcharacteristics during sudden acceleration.

[0088] The following embodiments are preferred for the devices disclosedin claims 10 and 11.

[0089] (1) The connecting passage is formed in the insulator gasketinterposed between the mounting surfaces of the insulator and cylinder,or in the carburetor gasket interposed between the mounting surfaces ofthe carburetor and the insulator. If one of these configurations isadopted, it will be possible to fashion a connecting passage merely bycreating a slit of the same diameter as the connecting passage in eitherthe insulator gasket or the carburetor gasket. Creating a connectingpassage in this way is straightforward and does not require a largenumber of processes.

[0090] (2) The connecting passage is cut into the surface of theinsulator where it is mounted to the cylinder.

[0091] (3) The connecting passage is cut into the surface of thecylinder where it is mounted to the insulator.

[0092] (4) The connecting passage is formed in the carburetor gasketwhich goes between the carburetor and the insulator.

[0093] (5) The connecting passage is cut into the surface of theinsulator where it is mounted to the carburetor.

[0094] (6) The connecting passage is cut into the surface of thecarburetor where it is mounted to the insulator.

[0095] (7) The connecting passage comprises a small hole in either theinsulator, the carburetor, or the cylinder, which connects the airpassage to the fuel passage.

[0096] (8) There is a non-return valve on the small hole which permitsflow only in the direction from the fuel passage to the air passage.

[0097] (9) The connecting passage is placed so that one end connects theair and fuel passages downstream from the non-return valve while theother end connects them upstream from the valve.

[0098] If the connecting passage is configured in this way, with one endbetween the air and fuel passages downstream from the non-return valve,the fuel-air mixture in the fuel passage can flow through the connectingpassage and into the air passage when the engine is mounted obliquely.This prevents fuel from collecting at the bottom of the fuel passage andbeing sucked into the cylinder suddenly when the position of the enginechanges, so it eliminates imperfect combustion due to an excess of fuel.The other end of the connecting passage connects the air and fuelpassages upstream from the non-return valve. This makes it possible toincrease the diameter of the outlet of the connecting passage without adrop in engine output, thus improving the engine's ability to acceleratesuddenly.

[0099] The invention disclosed in claim 12 of this application comprisesA two-stroke cycle engine with a scavenger passage which connects ascavenging port on the side of the cylinder to the crank chamber insidethe crankcase, and goes through the mounting surface where the cylinderand crankcase are attached to each other; and a removable guide with asurface forming a curved smooth channel which is attachable to thescavenger passage in the crankcase from the mounting surface, and formsa portion of the scavenger passage with the curved channel.

[0100] The guide should be configured as disclosed in claims 13 and 14.

[0101] The claim 13 comprises the configuration disclosed in claim 12.The guide has a positioning tooth which engages with the hole in thegasket for the surface where the cylinder and crankcase are attached toeach other.

[0102] The claim 14 comprises the configuration disclosed in claim 12.The guide is fixed to the crankcase when its tooth engages in anindentation in the crankcase.

[0103] If the engine is configured as described above, the fuel-airmixture from the crank chamber in the crankcase is led into a scavengerpassage one portion of which comprises a guide with a smoothly curvedchannel. The mixture flows through the scavenger passage formed as asmooth channel and is supplied to the scavenging port. Because thescavenger passage is a smoothly curved channel without any right angles,the fuel-air mixture flows smoothly and rapidly without any flow losssuch as a decrease in flow velocity as it is supplied to the scavengingport.

[0104] The guide is mounted on the crankcase in such a way that it canbe removed by pulling it away from the surface where the crankcase isattached to the cylinder along the axial direction of the cylinder. Thisobviates the need for a tooth between the upper wall of the scavengerpassage and the surface where the crankcase is attached, as was requiredin the prior art. The guide performs the same function as the tooth.When the crankcase is cast, even if a single die is used to form thescavenger passage inside the crankcase, the die can easily be removed inthe axial direction of the cylinder.

[0105] This design simplifies the casting procedure by which thescavenger passage is formed, and it reduces the number of castingprocesses required. Because the scavenger passage can be formed using asingle die, there is no possibility that one of several dies will slipout of position, as sometimes happened with prior art techniques, andruin the casting. This design, then, improves the quality of thecrankcase containing the scavenger passage.

[0106] If the engine is configured as disclosed in claim 13 of thisapplication, the guide has a positioning tooth which engages in a holein the gasket between the cylinder and crankcase. When the tooth engagesin the hole in the gasket, its position is assured, and the position ofthe gasket is also assured. The fact that the guide has been insertedcan be ascertained by how the gasket is positioned, so there is nochance that the guide will be forgotten.

[0107] If the engine is configured as disclosed in claim 14 of thisapplication, the crankcase has an indentation which serves as the slotinto which the tooth on the guide engages. Because the guide is fixed tothe crankcase, it will always be positioned correctly. The surface ofthe channel on the guide connects smoothly to the scavenger passage inthe crankcase.

[0108] Ideally, as is disclosed in the claim 15, which comprises theconfiguration disclosed in claim 13, the guide should have a depressionin the surface at which it is fixed to the crankcase.

[0109] If the engine is configured in this way, the slight gap betweenthe crankcase and cylinder where the gasket is interposed will besmaller. Less fuel will seep into the gap from the fuel-air mixtureflowing through the scavenger passage connecting the crankcase and thecylinder through the common surface where they are attached to eachother, which comprises the scavenger passage formed by the crankcase andthe guide, and the scavenger passage in the cylinder. The fuel flowsdownward through the depression formed in the guide, so even if theengine is mounted obliquely, the fuel cannot return to the scavengerpassage. This eliminates imperfect combustion due to fuel flowing backinto the scavenger passage.

[0110] Ideally, as is disclosed in the claim 16, which comprises theconfiguration disclosed in claim 12, the guide is distinguished by thefact that it is painted on.

[0111] If the guide is configured in this way, by being painted on, itwill always be perfectly plain whether or not it is there. This willeliminate the possibility that it will be forgotten during assembly. Byusing a different color for each type of machine, we can simplify ourparts control.

[0112] A preferred embodiment of the means disclosed in claim 12 of thisapplication is as follows.

[0113] The guide is formed by molding a deep-drawing sheet as a singlepiece with the gasket between the cylinder and crankcase. If produced inthis way, the guide and the gasket are one piece, and deep drawingallows the channel surface and the depression to be formed on the frontand reverse sides of the sheet at the same time. This reduces the partscount and the number of assembly processes.

[0114] The invention disclosed in claim 17 of this application comprisesa two-stroke cycle engine with an exhaust port on the sidewall of thecylinder, which opens into the cylinder; a scavenging port on thesidewall of the cylinder positioned a slight distance apart in thecircumferential direction from the exhaust port, which also opens intothe cylinder; an intake port, which opens to supply fuel-air mixture tothe crankcase according to the action of the piston; and a scavengerpassage, which connects the crankcase and the scavenging port; wherein ablow-up angle (α) of the scavenger passage, which is defined by an anglebetween the upper wall which connects to the scavenging port and aperpendicular line to the axis of the cylinder, varies along thecircumferential direction of the cylinder, and if the blow-up angle in alocation nearer the exhaust port is defined as (α1) and the blow-upangle in a location nearer the intake port is defined as (α2), thenα1<α2.

[0115] Ideally, as is disclosed in claim 18, which comprises theconfiguration disclosed in claim 17, the blow-up angle (α) of thescavenger passage varies continuously from a location nearer the intakeport to one nearer the exhaust port.

[0116] Also ideally, as is disclosed in claim 19, which comprises theconfiguration disclosed in claim 17, the surface of the upper wall ofthe scavenger passage is formed so that it varies in one or more stepsfrom angle (α2) at a location nearer the intake port to angle (α1) at alocation nearer the exhaust port.

[0117] If the scavenger passage is configured as described above, whenthe action of the piston causes the exhaust port and then the slightlylower scavenging port to open, the fuel-air mixture forced into thescavenger passage from the crankcase will flow from the scavenging portinto the combustion chamber.

[0118] Because the blow-up angle of the upper wall connecting thescavenger passage to the scavenging port is greater at a location nearerthe intake port than it is at a location nearer the exhaust port, thefuel mixture which enters the chamber from the location nearer theexhaust port will flow along the top of the piston at a high speedwithout being dispersed. This will prevent it from getting caught in theexhaust gas stream and so reduce the quantity of fuel lost through theexhaust port. The fuel-air mixture which enters the chamber from thelocation nearer the intake port will be flowing at a lower velocity thanthat nearer the exhaust port. It will be sent into the area around thespark plug in the upper part of the chamber, where it will beefficiently ignited and combusted.

[0119] Thus this configuration prevents the fuel-air mixture fromescaping unburned through the exhaust port, improves the scavengingefficiency, and increases the concentration of the fuel-air mixturewhich fills the combustion chamber. This improves the combustion andincreases the output of the engine. The fact that the fuel-air mixtureis prevented from escaping through the exhaust port translates into alower THC level (total hydrocarbons) in the exhaust.

[0120] If the engine is configured as disclosed in claim 19, the surfaceof the upper wall of the scavenger passage is formed so that it variesin step fashion from a large angle (α2) at a location nearer the intakeport to a smaller angle (α1) at a location nearer the exhaust port. Whenthe cylinder is cast, two dies can be used with two different blow-upangles, as described above, with the angles changing at the borderbetween the dies. This will make it easy to remove the work from thedies and will reduce the number of processes necessary to produce thecylinder. Also, using dies with two different blow-up angles to form thescavenger passage is an easy and reliable way to control the blow-upangle.

[0121] The invention disclosed in claim 20 of this application comprisesa two-stroke cycle engine with a scavenging port on the side of thecylinder, which opens into the cylinder; and a scavenger passage, whichconnects the crank chamber in a crankcase and the scavenging port, andsupplies the fuel-air mixture in the crank chamber to the scavengingport; wherein the crankcase is configured in such a way that the frontand rear portions, which are separated by a block at a right angle tothe crankshaft which entails the axis of the cylinder, are fixed to eachother by mounting hardware, a scavenger passage is provided inside boththe front and rear portions of the crankcase, and the cylinder, whosescavenger passage connects to the scavenger passage in the crankcase, isfixed by mounting hardware to the mounting surface on the top of thecrankcase in such a way that the scavenger passage runs through themounting surface.

[0122] The invention disclosed in claim 27 of this application comprisesa two-stroke engine with a scavenger passage which connects thecrankcase and the scavenging port on the side of the cylinder, whichopens into the cylinder and supplies the fuel-air mixture in thecrankcase to the scavenging port. This two-stroke engine isdistinguished by the following. In addition to having scavenger passagesin both the crankcase and the cylinder, the front and rear portions ofthe crankcase, separated by a block at a right angle to the crankshaft,which entails the axis of the cylinder, are fixed to each other at theblock by mounting hardware to form a unitary crankcase. The cylinder,whose scavenger passage connects to that in the crankcase, is fixed bymounting hardware to the mounting surface at the top of the crankcase.

[0123] Ideally, as is disclosed in claim 21, which comprises theconfiguration in claim 20, the air passage which supplies air from theair cleaner to the scavenger passage is formed inside the cylinder. Theair passage connects to the middle portion of the scavenger passageinside the cylinder.

[0124] If the engine is configured in this way, the crankcase isseparated by a block at a right angle to the crankshaft, and so to theaxis of the cylinder, into front and rear portions, each of which has ascavenger passage inside it. The front and rear portions of thecrankcase are fixed together by mounting hardware. The cylinder, whichalso has a scavenger passage inside it, is fixed to the upper surface ofthe crankcase by mounting hardware. The scavenger passage in thecylinder is thus linked to that in the crankcase, forming a longscavenger passage running through both cylinder and crankcase. Both thecrankcase and the cylinder are thus compact structures with no bulges,and the scavenger passage has a gradually curved contour withoutangularities.

[0125] Furthermore, a crankcase with a scavenger passage running throughits interior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to beforming a long scavenger passage running through both cylinder andcrankcase. Both the crankcase and the cylinder are thus compactstructures with no bulges, and the scavenger passage has a graduallycurved contour without angularities.

[0126] Furthermore, a crankcase with a scavenger passage running throughits interior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to forma long scavenger passage running through both cylinder and crankcase.Both the crankcase and the cylinder are thus compact structures with nobulges, and the scavenger passage has a gradually curved contour withoutangularities.

[0127] Furthermore, a crankcase with a scavenger passage running throughits interior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to becast using fewer dies. The casting procedure is simplified and requiresfewer processes.

[0128] Ideally, as is disclosed in claim 22, which comprises theconfiguration in claim 20, there should be two scavenging ports alongthe circumference of the cylinder. There should also be two scavengerpassages running from the outlets in the crankcase to the scavengingports. These passages should run through the block separating the halvesof the crankcase, and they should be arranged symmetrically along thefront-to-rear dimension of the engine.

[0129] If the engine is configured in this way, the scavenger passageswill run the entire length from the outlets of the crankcase to thescavenging ports in the cylinder and through the separator block, andthey will be symmetrical. Thus a common die can be used to cast thefront and rear portions of the scavenger passages, allowing the engineto be produced with only a few dies. The shapes of the two passages willbe identical, so the cylinder will be scavenged uniformly along itscircumference and filled uniformly with the fuel-air mixture.

[0130] The following embodiments are preferred for the devices disclosedin claims 20 through 22 of this application.

[0131] (1) The air passage branches into two passages at the inlet tothe cylinder. Each of these branching air passages extends from thebranch along the length of the engine, and the two run symmetricallythrough the block separating the halves of the crankcase. They areconnected to the scavenger passages.

[0132] If the engine is configured in this way, the two branching airpassages run in parallel through the block separating the halves of thecrankcase. Since they are identical, a common die can be used to casteach of the passages, and fewer dies need be used overall. Since theshapes of the two air passages are identical, the action of thescavenging air will be uniform along the circumference of the cylinder.

[0133] (2) The scavenger passages and branching air passages formed inthe cylinder are enclosed by walls which run virtually parallel to eachother in the same direction.

[0134] If configured in this way, the walls of the scavenging andbranching air passages are integral to the cylinder and run virtuallyparallel to each other. The sliding die for the cylinder can thus be asingle piece, which simplifies the configuration of the die.

[0135] The invention disclosed in claim 23 of this application comprisesa two-stroke engine with a scavenger passage which connects thecrankcase and the scavenging port on the side of the cylinder; an airpassage connected to the midpoint of the scavenger passage, whichsupplies scavenging air from the air cleaner to the scavenger passage;and a fuel passage, which supplies the fuel-air mixture produced in thecarburetor to the crankcase. This two-stroke engine is distinguished bythe following. The air cleaner has two air passages running from it inparallel, one of which is connected to the air passage, and the other ofwhich is connected to the air inlet of the carburetor to provide air forthe fuel-air mixture. A choke valve on the air cleaner opens and closesboth of these air passages.

[0136] Ideally, as is disclosed in claim 24, which comprises theconfiguration in claim 23, the choke valve comprises a rotary valvewhich, when rotated, opens or closes the openings of the two airpassages, and a knob by which the valve can be rotated.

[0137] Also, as is disclosed in claim 25, which comprises theconfiguration in claim 24, the choke of the choke valve engages with thecase of the air cleaner in such a way that it is free to rotate. Itsfront surface comprises a sheet which covers or uncovers the inlets ofthe two air passages. A sealing ring consisting of an elastic materialpresses the flat surface of the valve against the openings of the inletsby elastic force and forms a fluid seal around the valve shaft withrespect to the interior of the case.

[0138] A tapered protrusion is formed on the inner surface of the caseof the air cleaner. When the rotary knob of the choke valve strikes theprotrusion, the flat surface of the valve is pressed against the openingof either the first or the second of the two air passages.

[0139] If the choke is configured in this way, when the knob of thechoke valve is turned to start up the engine, the second air passage,which connects the air cleaner to the air inlet of the carburetor, iscompletely closed, and only the choke hole is open. The first airpassage, which connects the air cleaner to the air passage supplying thepreceding air, is also completely closed when the engine is started up.

[0140] When the choke is adjusted in this way, the air from the aircleaner which is supplied to the carburetor via the second air passageis constricted by the choke hole. The fuel-air mixture produced in thecarburetor is supplied via the crankcase, scavenger passage andscavenging port to the combustion chamber.

[0141] Since the first air passage from the air cleaner is completelyclosed by the choke at this time, the fuel-air mixture from thecarburetor is supplied to the combustion chamber without any precedingair. Thus the chamber is filled with a rich fuel-air mixture. Thisimproves the start-up characteristics of the engine.

[0142] The flat surface of the rotary choke of the choke valve coversand uncovers the openings of the two air passages. The elastic force ofthe O-ring or other sealing ring which is inserted around the shaft ofthe choke causes the flat surface to press against the openings. Thechoke valve thus completely and reliably closes the opening of thepassage for the preceding air. This promotes the production of a richfuel-air mixture as described above and allows a high negative pressureto be maintained.

[0143] This invention provides both a sealing ring as described aboveand a protrusion on the outside of the case of the air cleaner to serveas a stop for the choke, and rotating the choke valve from its initialposition to its normal operating position will switch between the twoaforesaid air passages. When the choke valve is rotated, its knob easilygoes over the protrusion against the force of the sealing ring. Themoderate friction improves the operating feel of the choke valve. Whenthe choke valve is released, the elastic force of the sealing ring andthe force of the protruding stop automatically hold the choke valve inplace on the flat portion of the case in such a way that it cannot goback. This insures easy operation.

[0144] There is, as is disclosed in claim 25, a tapered protrusion onthe inner surface of the case of the air cleaner. When the choke valveis rotated toward starting position, the knob of the choke valve goesover the protrusion. When this occurs, the elastic force generated bythe deflection of the valve causes its flat surface to push down on theopening of the passage for the preceding air. This improves the sealingfunction of the flat surface, and thus improves the start-upcharacteristics.

[0145] The invention disclosed in claim 26 of this application is atwo-stroke engine which is distinguished by the following. It has arotary valve installed on the case in such a way that it is free torotate which, when rotated, opens and closes the two air passages; and arotary knob which operates the valve. The front surface of the valvecomprises a sheet which covers or uncovers the inlets of the two airpassages. A sealing ring consisting of an elastic material presses theflat surface of the valve against the openings of the inlets by elasticforce and forms a fluid seal around the valve shaft with respect to theinterior of the case.

[0146] A rotary valve configured in this way is not limited in itsapplication to use as a choke valve for the air cleaner of a two-strokeengine. It can be used in a wide range of applications which requireswitching between two fluid passages by operating a rotary valve.

[0147] To summarize, the effects of the invention disclosed above are asfollows.

[0148] (1) With the inventions disclosed in claims 1 through 8 of thisapplication, all scavenger passages and air passages are formed insidethe crankcase and the cylinder. This obviates the need for externalpipes to serve as air passages as well as their mounting hardware. Fewerparts and assembly processes are required, and the engine can be madelighter and smaller.

[0149] The scavenger passage is formed of two passages, one created bywalls in the crankcase and the other by similar walls in the cylinder.The result is a long scavenger passage which can be filled with air forthe scavenging operation. Since the fuel-air mixture is supplied onlyafter the crankcase has been thoroughly scavenged by this air, no fuelis lost through the exhaust port, and fuel wastage is minimized.

[0150] If configured as disclosed in claim 2, the outlet of thescavenger passage on the side of the crankcase opens into the crankchamber. The gap between the side of the crankcase and the crank web isreduced, and a disk valve is formed by the outlet on the crankcase andthe crank web. This controls the velocity of the air forced in throughthe scavenging port and reduces the quantity of fuel-air mixture whichbecomes trapped in the exhaust gas stream.

[0151] If configured as disclosed in claim 4, the branching air passageand the branching passage to the scavenging port in the cylinder areboth enclosed by walls that run parallel in the same direction. Thisallows the sliding die for the scavenger passage to be one piece in theprocess of casting the cylinder. The die can have a simpler shape andwill be cheaper to produce.

[0152] If the engine is configured as disclosed in claims 5 through 8, avalve controls the air flow supplied to the combustion chamber via thescavenging port of the engine. This air control valve is interlockedwith the fuel-mixture control valve through a cam mechanism whichrotates with the fuel-mixture valve. This allows the opening and closingof the air and fuel control valves to be controlled in relation to eachother.

[0153] Thus when the engine is operating under a light load, the aircontrol valve can be fully closed or opened only slightly so that thefuel mixture is richer and stable combustion can be maintained. When theengine is operating under a heavier load, the air control valve can beopened or closed proportionally with the fuel mixture control valve toproduce a thinner mixture. We can thus provide an air-layer-typescavenging two-stroke engine in which the overall richness of themixture can be kept at the appropriate concentration, the rate of fuelconsumption is reduced, and the quantity of pollutants in the exhaustgas is lowered.

[0154] The opening of the mixture control valve which controls the flowrate of the fuel-air mixture is interlocked via a cam mechanism to theopening of the air control valve. The angular ratio of the mixturecontrol valve to the air control valve can be set as desired; it willremain constant regardless of the angular position of the mixturecontrol valve. This design, then, allows the user to select the mostadvantageous ratio.

[0155] (2) With the invention disclosed in claim 9, the air passage andthe fuel mixture passage are linked by a small-diameter connectingpassage. Thus the fuel mixture in the fuel passage can be supplied viathis small-diameter connecting passage to the air passage. In this waymore fuel-air mixture can be added to the air flowing through the airpassage. This prevents the new air supplied to the cylinder from thescavenging port from creating too thin a mix when the engine acceleratessuddenly, and thus improves the acceleration characteristics.

[0156] When the engine is operating at high speed, there is virtually nopressure differential between the fuel passage and the air passage.There will therefore be almost no fuel-air mixture flowing via thesmall-diameter connecting passage from the fuel passage to the airpassage, and thus no fuel in the layer of preceding air. This willinsure that the required exhaust specifications can be maintained.

[0157] When the engine is mounted obliquely, the fuel-air mixture in thefuel passage can flow into the air passage via the connecting passage.This prevents fuel from collecting in the lowest portion of the fuelpassage so that a large quantity of fuel is suddenly sucked into thecylinder when the engine's orientation is changed. This design, then,will prevent the imperfect combustion which would occur if there wereexcess fuel in the chamber.

[0158] Providing a small-diameter connecting passage between the air andfuel passages obviates the need for a control device with a complicatedconfiguration. It allows us to realize an air-layer-type scavengingtwo-stroke engine which can achieve the same effect with an extremelysimple device.

[0159] One end of the connecting passage connects the air and fuelpassages downstream from the non-return valve. The effect of this whenthe engine is mounted obliquely is that it will prevent an excess offuel in the combustion chamber which will result in imperfectcombustion. The other end of the connecting passage connects the air andfuel passages upstream from the non-return valve. This makes it possibleto increase the diameter of the outlet of the connecting passage withouta drop in engine output, thus improving the engine's ability toaccelerate suddenly.

[0160] (3) With the invention disclosed in claim 12, a surface of aguide which forms the shape of a channel connects smoothly with thescavenger passage in the crankcase. The resulting scavenger passage isgradually curved with no right angles, so the fuel-air mixture whichmoves through it does not experience any loss of flow throughdeceleration, but is supplied to the scavenging port smoothly and at ahigh velocity. This improves the engine output.

[0161] The guide is mounted on the crankcase in such a way that it canbe removed by pulling it away from the surface where the crankcase isattached to the cylinder along the axial direction of the cylinder. Theguide fulfills the function of the projection that was used in the priorart. When the crankcase is cast, even if a single die is used to formthe scavenger passage inside the crankcase, the die can easily beremoved in the axial direction of the cylinder.

[0162] This design simplifies the casting procedure by which thescavenger passage is formed, and it reduces the number of castingprocesses required. Because the scavenger passage can be formed using asingle die, there is no possibility that one of several dies will slipout of position, as sometimes happened with prior art techniques, andruin the casting. This design, then, improves the quality of thecrankcase containing the scavenger passage.

[0163] If the engine is configured as disclosed in claim 13, the guidehas a positioning tooth which engages with the hole in the gasket. Whenthe tooth engages in the gasket hole, its position is guaranteed, and sois that of the gasket. The fact that the guide is in place can be knownfrom how the gasket is seated, so there is no chance of forgetting theguide.

[0164] If the engine is configured as disclosed in claim 14, the guideis fixed to the crankcase when its tooth engages in an indentation inthat crankcase. In this way the guide can be positioned with completeaccuracy so that its surface which forms a channel can connect smoothlyto the scavenger passage in the crankcase.

[0165] If the engine is configured as disclosed in claim 15, even if thefuel in the fuel-air mixture flowing through the scavenger passageconnecting the crankcase and the cylinder through the common surfacewhere they are attached to each other seeps into the gap between thesurfaces where the gasket is inserted, the fuel will flow downwardthrough the depression formed in the guide. Thus even if the engine ismounted obliquely, the fuel cannot return to the scavenger passage. Thiseliminates imperfect combustion due to fuel flowing back into thescavenger passage.

[0166] (4) If the engine is configured as disclosed in claim 17, theblow-up angle of the upper wall of the scavenger passage which connectsit to the scavenging port is greater at a location nearer the intakeport than at one nearer the exhaust port. The fuel mixture which entersthe chamber from the location nearer the exhaust port will flow alongthe top of the piston at a high speed without being dispersed. This willprevent it from getting caught in the exhaust gas stream and so reducethe quantity of fuel lost through the exhaust port. The fuel-air mixturewhich enters the chamber from the location nearer the intake port willbe flowing at a lower velocity than that nearer the exhaust port. Itwill be sent into the area around the spark plug in the upper part ofthe chamber, where it will be efficiently ignited and combusted. Thescavenging efficiency is improved, the fuel-air mixture which fills thecombustion chamber has a higher concentration, and the combustion isimproved, resulting in greater engine output. Also, preventing thefuel-air mixture from escaping reduces the level of THC (totalhydrocarbons) in the exhaust.

[0167] If the engine is configured as disclosed in claim 19, the surfaceof the upper wall of the scavenger passage is formed so that it variesin step fashion from a large blow-up angle at a location nearer theintake port to a smaller blow-up angle at a location nearer the exhaustport. When the cylinder is cast, two dies can be used with two differentblow-up angles, with the angles changing at the border between the dies.This will make it easy to remove the work from the dies and will reducethe number of processes necessary to produce the cylinder.

[0168] Also, using dies with two different blow-up angles to form thescavenger passage is an easy and reliable way to control the blow-upangle.

[0169] (5) With the inventions disclosed in claims 20 and 27 of thisapplication, the crankcase is divided into front and rear portions, eachof which has a scavenger passage inside it. These two portions of thecrankcase are fixed to each other by mounting hardware. The cylinder,whose scavenger passage connects to that in the crankcase, is fixed bymounting hardware to the mounting surface at the top of the crankcase.This results in a long scavenger passage running through both cylinderand crankcase. Both the crankcase and the cylinder are thus compactstructures with no bulges, and the scavenger passage has a graduallycurved contour without angularities.

[0170] Furthermore, a crankcase with a scavenger passage running throughits interior can be cast in two pieces which form the front and rearportions of the engine. These can be removed from the die at the surfacewhere the front and rear pieces are separated and at the surface wherethe cylinder is mounted to the crankcase, which is perpendicular to thesurface between the pieces. This allows the die to have a simple shape,simplifies removing the work from the die, and allows the engine to becast using fewer dies. The casting procedure is simplified and requiresfewer processes, with the result that the cost is lower.

[0171] If the engine is configured as disclosed in claim 22, thescavenger passages will run the entire length from the outlets of thecrankcase to the scavenging ports in the cylinder and through thesurfaces where the two portions meet, and they will be symmetrical. Thusa common die can be used to cast the front and rear portions of thescavenger passages, allowing the engine to be produced with fewer dies.The shapes of the two passages will be identical, so the cylinder willbe scavenged uniformly along its circumference and filled uniformly withthe fuel-air mixture.

[0172] Furthermore, the two branching air passages run symmetricallythrough the surface between the two halves of the crankcase. If thepassages are configured in this way, a common die can be used to castboth of them, so fewer dies need be used overall. Since the shapes ofthe two air passages are identical, the action of the scavenging airwill be uniform along the circumference of the cylinder.

[0173] Also, the walls of the scavenging and branching air passages canbe formed integrally to the cylinder and virtually parallel with eachother. If configured in this way, the sliding die for the cylinder canbe a single piece. This simplifies the configuration of the die andreduces the cost of producing it.

[0174] (6) With the invention disclosed in claim 23, the first airpassage from the air cleaner is completely and reliably closed by thechoke valve when the engine is being started up. Thus the supply ofscavenging air to the combustion chamber is cut off, and only thefuel-air mixture from the carburetor is supplied to the combustionchamber. Thus the chamber is filled with a rich fuel-air mixture. Thisimproves the start-up characteristics of the engine.

[0175] The flat surface of the rotary choke of the choke valve coversand uncovers the openings of the two air passages. The elastic force ofthe O-ring or other sealing ring which is inserted around the shaft ofthe choke causes the flat surface to press against the openings. Ifconfigured in this way, the choke valve completely and reliably closesthe opening of the passage for the preceding air. This promotes theproduction of a rich fuel-air mixture as described above and allows ahigh negative pressure to be maintained.

[0176] In addition to the sealing ring, a protrusion is provided on theoutside of the case of the air cleaner to serve as a stop for the choke.If the choke valve is configured in this way, rotating it from itsinitial position to its normal operating position will switch betweenthe two aforesaid air passages. When the choke valve is rotated, itsknob easily goes over the protrusion against the force of the sealingring. The moderate friction improves the operating feel of the chokevalve. When the user releases the choke valve after operating it, theelastic force of the sealing ring and the force of the protruding stopautomatically hold the choke valve in place on the flat portion of thecase in such a way that it cannot go back. This insures easy operation.

[0177] Furthermore, as is disclosed in claim 25, there is a taperedprotrusion on the inner surface of the case of the air cleaner. When thechoke valve is rotated toward starting position, the knob of the chokevalve goes over the protrusion. When this occurs, the elastic forcegenerated by the deflection of the valve causes its flat surface to pushdown on the opening of the passage for the preceding air. This improvesthe sealing function of the flat surface, and thus improves the start-upcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0178]FIG. 1 is a cross section taken at a right angle to the crankshaftwhich indicates the center passage of the cylinder in an air-layer-typescavenging two-stroke engine which is the first preferred embodiment ofthis invention.

[0179]FIG. 2 is a perspective drawing showing the arrangement of thescavenging and air passages in the first preferred embodiment.

[0180]FIG. 3 corresponds to FIG. 2, and shows the operation of theengine which is the first preferred embodiment.

[0181]FIG. 4 shows the relationship between the crank web and thescavenger passage in the first preferred embodiment.

[0182]FIG. 5 is a cross section taken along passage A-A in FIG. 1.

[0183]FIG. 6 is a cross section of the air control valve and itssurrounding area in the second preferred embodiment of this invention.

[0184]FIG. 7 is a horizontal cross section of the air control valve inthe second preferred embodiment of this invention.

[0185]FIG. 8 is the view of the air control valve seen from line B-B inFIG. 6.

[0186]FIG. 9 is a graph showing an example of the relationship betweenthe throttle valve and the air control valve.

[0187]FIG. 10 corresponds to FIG. 6 and shows the third preferredembodiment of this invention.

[0188]FIG. 11 is a cross section which corresponds to FIG. 7 and showsthe third preferred embodiment of this invention.

[0189]FIG. 12 eighth preferred embodiment of this invention.

[0190]FIG. 13 is a frontal view of the carburetor gasket in the fourthpreferred embodiment of this invention.

[0191]FIG. 14 is a view of the end of the carburetor gasket on theinsulator (from line A0-A0 in FIG. 12) in the fourth preferredembodiment of this invention.

[0192]FIG. 15 is a magnified cross section of the area where theinsulator and cylinder are connected (an enlargement of portion Z inFIG. 12) in the fourth preferred embodiment of this invention.

[0193]FIG. 16 is a plan view of the insulator gasket in the fourthpreferred embodiment of this invention.

[0194]FIG. 17 is a view of the end of the insulator gasket on theinsulator (from line B0-B0 in FIG. 15) in the fourth preferredembodiment of this invention.

[0195]FIG. 18 is a view of the connecting passage in the fourthpreferred embodiment or this invention which corresponds to FIG. 15.

[0196]FIG. 19 is a cross section taken at a right angle to thecrankshaft and showing the scavenger passage in a two-stroke enginewhich is the fifth preferred embodiment of this invention.

[0197]FIG. 20 is the view from line A1-A1 in FIG. 19.

[0198]FIG. 21 is a frontal view of the guide in the fifth preferredembodiment.

[0199]FIG. 22 is the view from arrow B1 in FIG. 19.

[0200]FIG. 23 is a cross section taken along the axis of the cylinderand showing the cylinder in an air layer-type two-stroke engine which isthe sixth preferred embodiment of this invention.

[0201]FIG. 24 is a view of the seventh preferred embodiment of thisinvention which corresponds to that in FIG. 23.

[0202]FIG. 25 are cross sections of the scavenger passages in thecylinders of the sixth and seventh preferred embodiments. (A) is takenalong line A2-A2 in FIGS. 23 and 24; (B) is taken along line B2-B2.

[0203]FIG. 26 is a perspective drawing of the scavenging port in theseventh preferred embodiment.

[0204]FIG. 27 is a cross section taken along the axis of the cylinder inan air layer-type scavenging two-stroke engine in which the sixth andseventh preferred embodiments of this invention have been implemented.

[0205]FIG. 28 is an exploded perspective drawing of an air layer-typescavenging two-stroke engine which is the eighth preferred embodiment ofthis invention.

[0206]FIG. 29 is a cross section taken along the crankshaft of atwo-stroke engine which is the eighth preferred embodiment of thisinvention.

[0207]FIG. 30 shows the configuration of the scavenging and air passagesin the eighth preferred embodiment of this invention.

[0208]FIG. 31 is a cross section taken at a right angle to thecrankshaft which describes the axis of the cylinder in an air layer-typescavenging two-stroke engine in which this invention has beenimplemented.

[0209]FIG. 32 is a cross section taken at a right angle to thecrankshaft which shows the configuration of the air cleaner and vacuumdevice in a two-stroke engine which is the ninth preferred embodiment ofthis invention.

[0210]FIG. 33 is the view from arrow 3 in FIG. 32.

[0211]FIG. 34 is the view from line B3-B3 in FIG. 32.

[0212]FIG. 35 is the view from line C3-C3 in FIG. 32.

[0213]FIG. 36 is a cross section of the air cleaner cover and chokevalve (a closer view of area Z in FIG. 32).

[0214]FIG. 37 is the view from line D3-D3 in FIG. 36.

[0215]FIG. 38 is a cross section taken along line E-E in FIG. 37.

[0216]FIG. 39 is an example of the prior art which corresponds to FIG.19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0217] In this section we shall give a detailed explanation of a numberof preferred embodiments of this invention with reference to thedrawings. Whenever the shapes, relative positions and other aspects ofthe parts described in the embodiments are not clearly defined, thescope of the invention is not limited only to the parts shown, which aremeant merely for the purpose of illustration.

[0218]FIGS. 1 through 5 show the first preferred embodiment of thisinvention. 2 is the cylinder, 4 the piston, 6 the crankshaft, 6 a thecrank web comprising the crankshaft 6, 5 the crankcase, 3 the connectingrod which connects piston 4 and crankshaft 6, 7 the cylinder head, 8 thespark plug, 11 the air cleaner and 12 the carburetor. 25 is thecombustion chamber. 5 a is the crank chamber formed inside crankcase 5.15 b and 15 are the passage for the fuel-air mixture which connects thecarburetor 12 and crank chamber 5 a. 13 a is the exhaust port whichopens into the side of cylinder 2. It is connected to exhaust pipe 13.

[0219]9 a are two scavenging ports to the right and left of exhaust port13 a on cylinder 2, oriented at virtually a right angle to exhaust port13 a. As can be seen in FIG. 2, the scavenging ports 9 a are connectedto the crank chamber 5 a via two branching scavenger passages 109 e,which are mounted obliquely on cylinder 2; two scavenger passages 109 f,which are at the point where the various scavenger passages meet;arc-shaped scavenger passages 109 d, which are enclosed by walls oneither side of crankcase 5; and outlets 109 b.

[0220] As can be seen in FIG. 5, the end surfaces 109 g of the outlets109 b and the end surfaces 6 d of crank webs 6 approximate each other inthe direction of crankshaft 60, leaving only a microscopic gap, so thatthe ends of the outlets can be opened and closed by the action of crankwebs 6 a of crankshaft 6. At a right angle to crankshaft 60, as can beseen in FIG. 4, the outlets 109 b of the scavenger passages areprogressively uncovered by crank webs 6 a as crankshaft 6 rotates indirection N. The lower portions of outlets 109 b 6 a are tapered so thatinitially a small part of the openings is uncovered and thenprogressively a larger and larger portion of them.

[0221]10 is an air supply chamber in the side of cylinder 2. Itsupstream side is connected to air passage 10 b in insulator 30, whichwill be discussed shortly. Its downstream side is connected to the twobranching air passages 10 a. The branching air passages 10 a, as isshown in FIG. 2, connect to scavenger passages 109 f and branchingscavenger passages 109 e.

[0222] The air supply chamber 10 has non-return valves 16 on the outletsto branching air passages 10 a on its right and left, which permit airto flow only toward branching air passages 10 a.

[0223] As can be seen in FIGS. 2 and 3, the branching air passages 10 aand scavenger passages 109 e are formed virtually symmetrically withrespect to the axis 50 of the cylinder by walls 109 h and 109 i, whichextend outward from the sides of the cylinder 2 and are integral to it.The walls 109 h and 109 i are parallel to each other. The single diewhich forms them can be removed by pulling it sideward away from thecylinder.

[0224]30 is an insulator to thermally isolate the engine body from thevacuum system. The insulator 30 is bolted to the side of cylinder 2. Theair passage 10 b is in the upper portion of the insulator 30, and fuelpassage 15 b is in its lower portion.

[0225] The upstream side of the fuel passage 15 b is connected tocontrol valve 14 on the carburetor, which controls the rate of flow ofthe fuel-air mixture. The downstream side is connected to the interiorof the cylinder (combustion chamber 25) via fuel supply inlet 15 a.

[0226]110 is an air passage which is integral to carburetor 12. Itconnects air cleaner 11 and insulator 30. Control valve 20 varies thesize of the opening of the air passage 110. The control valve 20 isinterlocked with mixture control valve 14 on carburetor 12.

[0227] When an air layer-type scavenging two-stroke engine configured inthis way operates, the explosive force inside combustion chamber 25pushes piston 4 downward and opens exhaust port 13 a. The combustiongases (i.e., the exhaust gas) in the chamber 25 go out exhaust port 13 ainto exhaust pipe 13 and are released to the exterior through themuffler (not pictured).

[0228] When piston 4 goes down further, the scavenging ports 9 a to itsleft and right open. The air which has accumulated in branchingscavenger passage 109 flows into combustion chamber 25, as indicated bythe arrow in FIG. 3, and pushes the combustion gases toward exhaust port13 a.

[0229] Next, the fuel-air mixture stored in crank chamber 5 a flows intocombustion chamber 25 via outlets 109 b of the scavenger passages,scavenger passages 109 d and branching scavenger passages 109 e.

[0230] When piston 4 goes down, as can be seen in FIG. 3, and reachesbottom dead center, exhaust port 13 a and scavenging ports 9 a open, andthe supply of air and fuel-air mixture to combustion chamber 25 iscompleted or attempts to be completed. When piston 4 rises from bottomdead center, it closes scavenging ports 9 a, and the interior of crankchamber 5 a becomes a closed space. As the space begins to expand, itspressure begins to drop.

[0231] When piston 4 rises further, exhaust port 13 a closes, and thefuel-air mixture in combustion chamber 25 begins to be pressurized. Aspiston 4 rises, the volume inside crank chamber 5 a increases, whichfurther reduces the pressure in the crankcase. As can be seen in FIG. 2,when piston 4 rises further, fuel supply inlet 15 a on the side ofcylinder 2 opens, and the fuel-air mixture generated in carburetor 12and controlled by valve 14 is supplied to crank chamber 5 a through fuelpassages 15 b and 15, as indicated by the arrows in FIG. 2.

[0232] The drop in pressure inside the crank chamber 5 a is communicatedvia outlets 109 b, scavenger passages 109 d and branching scavengerpassages 109 e to branching air passages 10 a on the left and right.Non-return valve 16 opens, and the air supplied to air supply chamber 10via valve 16 through a process we shall discuss shortly flows into crankchamber 5 a.

[0233] The various pairs of passages which run from the scavenging ports9 a to crank chamber 5 a, namely, branching scavenger passages 109 e,and scavenger passages 109 f and 109 d, together form two long scavengerpassages. The air supplied to the scavenger passages must fill theirentire length before it is admitted into crank chamber 5 a.

[0234] When piston 4 reaches the vicinity of top dead center, spark plug8 discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

[0235] When piston 4 goes down and exhaust port 13 a opens, thecombustion gases in combustion chamber 25 flow through exhaust port 13 ainto exhaust pipe 13. They are exhausted to the exterior through themuffler (not pictured).

[0236] When piston 4 begins to drop, the gases in crank chamber 5 a arepressurized by its reverse side. When piston 4 drops further, theoutlets of scavenging ports 9 a on either side of it open. The fuel-airmixture supplied to crank chamber 5 a as described above is sucked intocombustion chamber 25 from scavenging ports 9 a via outlets 109 b,scavenger passages 109 d and 109 f, and branching scavenger passagess109 e. The combustion gases (i.e., the exhaust gas) in the combustionchamber 25 is pushed out through exhaust port 13 a in the scavengingoperation.

[0237] When the chamber is to be scavenged, the non-return valve 16described above is opened, and an appropriate quantity of air is allowedto fill scavenger passages 109 d and 109 f and branching scavengerpassages 109 e. Thus at the completion of a specified time interval fromthe beginning of scavenging, everything between scavenging ports 9 a andcombustion chamber 25 will have been scavenged by air. Only then is thefuel-air mixture in crank chamber 5 a forced into combustion chamber 25from scavenging ports 9 a via scavenger passages 109 d and branchingpassages 109 e.

[0238] When the process is executed repeatedly, the engine operates andpower is generated.

[0239] As is shown in FIGS. 1 through 3, in this sort of preceding airlayer-type scavenging two-stroke engine, scavenger passages 109 d incrankcase 5 are symmetrical with respect to the axis 50 of the cylinderon either side of the crankcase, and scavenger passage outlets 109 bopen into crank chamber 5 a symmetrically with respect to axis 50 of thecylinder.

[0240] And as can be seen in FIG. 5, crank webs 6 a form disk valvesover outlets 109 b into crankcase 5, with a small gap between the ends109 g of the outlets 109 b and the ends 6 d of crank webs 6 a.

[0241]FIG. 4 shows how an outlet 109 b is progressively uncovered when acrank web 6 a rotates in crank chamber 5 a. The position of the outlets109 b varies along the direction of rotation N of crankshaft 6. Thetiming at which the scavenging air is forced through scavenging ports 9a also varies. As can be seen in FIG. 4, the openings of outlets 109 bare tapered so that their size progressively increases as crank webs 6 arotate. By controlling the velocity of the scavenging air entering viascavenging ports 9 a, we can further reduce the quantity of fuel-airmixture which gets caught in the exhaust gas stream.

[0242] In the preceding air layer-type scavenging two-stroke enginewhich is the first preferred embodiment of this invention, branching airpassages 10 a on cylinder 2 and branching scavenger passages 109 e toscavenging ports 9 a on cylinder 2 are surrounded by parallel walls 109h and 109 i, as can be seen in FIG. 2. The sliding die used to cast thecylinder can thus be a single piece. The configuration of the die issimpler, and this reduces its cost.

[0243]FIGS. 6 through 8 show the second preferred embodiment of apreceding air layer-type scavenging two-stroke engine according to thisinvention. In this embodiment, the air control valve 20 of thetwo-stroke engine of the first embodiment is replaced by a modifiedvalve 20 which has the following improvements.

[0244] In FIGS. 6 through 8, 11 is the air cleaner, 12 the carburetor,10 e the air passage in carburetor 12, and 14 the fuel mixture controlvalve of carburetor 12. 15 and 15 b are the fuel mixture passages, and15 a is the supply inlet for the fuel mixture on cylinder 2.

[0245]20 is the air control valve. 45 is an air pipe connecting the airpassage 10 e of the air cleaner 11 to a separate outlet. 35 is the airsupply pipe in the insulator attached to the cylinder 2. 36 is a jointpipe for intake air. It is attached to air supply pipe 35 in theinsulator and connected to the outlet of the air passage 110.

[0246] As is shown in FIG. 7, the scavenging air, which is conductedthrough a different outlet on the air cleaner 11 than is the air passage10 e, flows through air passage 110 in air pipe 45. It goes through airpassage 10 b, which is surrounded by the joint pipe 36 for air intakeand air supply pipe 35 of the insulator. It passes from air valve 37 ofcontrol valve 20, which will be discussed shortly, and valve seat 35 a.It flows through air supply chamber 10 in cylinder 2 by way ofnon-return valve 16, and is sent to branching air passage 10 a, as shownin FIG. 1.

[0247]20 is an air control valve. The valve 20 is umbrella-shaped and isinstalled on joint pipe 36 for air intake. Air valve 37, which isattached to the end of the valve stem 39 in such a way that it can movealong the stem, engages in valve seat 35 a in such a way that it canfreely be attached or removed. Cam follower 38 is on the end of thevalve stem 39 which juts out from joint pipe 36. Compressive spring 41exerts force in the direction which is downstream in terms of the airflow through the air valve 37, that is, it exerts pressure on air valve37 against valve seat 35 a.

[0248] Portion 124 c is bent downward on the end of control lever 124,which is attached to the rotary shaft of fuel mixture control valve 14of the carburetor 12. Cam 124 a, which has a fan-shaped cross section,is formed on the bent portion 124 c. The internal cam 124 a engages withcam follower 38, which is on the other end of the air valve 37, to openand close that valve. The internal cam 124 a engages with the camfollower 83 so as to open air valve 37 in the direction opposite theflow of air, through the cam follower 38, against the force ofcompressive spring 41.

[0249] As is well known, the control lever 124 adjusts the opening ofmixture control valve 14; i.e., it adjusts the rate of flow of thefuel-air mixture. Swivel 125 is mounted so that it can rotate onoperating end 124 b of the side of the control lever 124 which isopposite the internal cam 124 a. Control lever 124 is operated bycontrol cord 142, which is connected from the exterior to the swivel125.

[0250] In the second preferred embodiment, the pressure in crank chamber5 a drops when piston 4 rises. When a negative pressure is achieved, itis communicated from crank chamber 5 a to air supply chamber 10 viascavenger passages 109 b, 109 d and 109 f and branching air passages 10a. When the air supply chamber 10 goes to negative pressure, non-returnvalve 16 opens.

[0251] When the control lever 124 is operated, internal cam 124 arotates and cam follower 38 of air control valve 20 is pulled leftwardin FIGS. 6 and 7 against the spring force of compressive spring 41. Thisopens air valve 37. Scavenging air from air cleaner 11 flows into airsupply chamber 10 through air pipe 45, air passages 110 and 10 a, airvalve 37 and non-return valve 16.

[0252] From the air supply chamber 10, just as in the first embodiment,the scavenging air flows into crank chamber 5 a by way of branching airpassages 10 a, scavenger passages 109 f and 109 d and scavenger passageoutlets 109 b as indicated by arrows in FIGS. 2 and 3. It accumulates inthe passages and the crank chamber 5 a, and scavenges them in the sameprocess as in the first embodiment.

[0253] Generally, in two-stroke engines, mixture control valve 14 willbe adjusted under a partial load to control the rate of flow of thefuel-air mixture supplied to combustion chamber 25 through crank chamber5 a. In this way a nearly constant concentration of fuel-air mixture canbe maintained over a wide range of operation.

[0254] In the air layer-type scavenging two-stroke engine related tothis invention, as has been discussed above, in the scavenging processair is supplied to combustion chamber 25 through air supply chamber 10instead of fuel-air mixture. If the concentration of the fuel-airmixture has a given concentration, and a quantity of air proportional tothe quantity of fuel-air mixture is supplied to combustion chamber 25,the overall richness of the mixture can be kept constant.

[0255] In the second preferred embodiment of this invention, mixturecontrol valve 14, which controls the flow rate of the fuel-air mixture,and air control valve 20, which controls the flow rate of the scavengingair sent into combustion chamber 25 through air supply chamber 10, areinterlocked through internal cam 124 a of lever 124, which adjustsmixture control valve 14, and cam follower 38. By selecting the profileof internal cam 124 a, we can attain an appropriate ratio for the degreeof openness of mixture control valve 14 and air control valve 20 (i.e.,an appropriate ratio of the flow rate of the fuel-air mixture to that ofthe scavenging air).

[0256] The graph shown in FIG. 9 is an example of an appropriaterelationship between the degree of opening of mixture control valve 14and air control valve 20. In this figure, R1, which is shown by a solidline, is an example in which combustion is stabilized under light loadconditions, including idling, which occur until valve 14 reaches itsfixed Z point of opening, by supplying a relatively rich mixture. Inthis region, i.e., the region in which valve 14 has not yet reached itsZ point, air control valve 20 is completely closed. Beyond the Z point,the degree of openness of valves 14 and 20 are set proportionally toeach other. R2, which is shown by a broken line, is an example in whichair control valve 20 is less open than in R1 while mixture control valve14 is only partly open.

[0257] In the second preferred embodiment, mixture control valve 14,which controls the flow rate of the fuel-air mixture, and air controlvalve 20 are interlocked through internal cam 124 a, which is connectedto the control lever 124, and cam follower 38. Thus the opening ratio ofthe mixture control valve 14 and air control valve 20 can easily be setin response to a change in the angular position of mixture control valve14. In other words, as can be seen in FIG. 9, while the engine is undera light load, as when idling, until it reaches the Z point, air controlvalve 20 is completely closed. Fuel-air mixture whose flow rate iscontrolled by mixture control valve 14 is sent into combustion chamber25 to fill the chamber both for scavenging and combustion.

[0258] Once the opening of mixture control valve 14 reaches the Z pointin FIG. 9, internal cam 124 a of control lever 124 draws air valve 37out to open it, and air control valve 20 moves into the open position.

[0259] When the load on the engine increases and mixture control valve14 opens further, air control valve 20 operates with the rotation ofinternal cam 124 a of control lever 124, which follows the rotation ofvalve 14, and opens proportionally to the valve 14. The flow rate of airincreases, and the engine is run with the weak mixture that a heavy loaddemands.

[0260] Other aspects of the configuration are identical to correspondingaspects of the first preferred embodiment, pictured in FIGS. 1 through5. The same parts have been numbered in the same way as in thesefigures.

[0261]FIGS. 10 and 11 show an air layer-type scavenging two-strokeengine which is the third preferred embodiment of this invention. Thisembodiment concerns the valve which controls the quantity of scavengingair and its operating mechanism, as in the second embodiment. It differsfrom the second embodiment in that the internal cam on control lever 124is formed at a given height on the inner edge of the fan-shapedperiphery of the control lever.

[0262] In FIGS. 10 and 11, the scavenging air conducted out of aircleaner 11 through a different outlet than the one to air passage 10 epasses through air passage 10 b, which is surrounded by joint pipe 48and intake pipe 47 in the insulator. It flows through the opening of airvalve 37 and valve seat 47 a in air control valve 20. It goes throughnon-return valve 16 and air supply chamber 10 in cylinder 2 and is sentto branching air passage 109 (See FIG. 1).

[0263] Control lever 124 is fixed to the rotary stem of mixture controlvalve 14 on the carburetor 12. On the end of control lever 124, incontrast to the configuration of the second embodiment, is portion 127c, which is bent upward. This forms internal cam 127 a, whose horizontalcross section is fan-shaped. The internal cam 127 is interlocked withcam follower 38 of air valve 37 so that it opens and closes that airvalve. The internal cam 127 a is installed so that it opens air valve 37through the action of cam follower 38 in the direction opposite the airflow, i.e., in the direction in which it exerts pressure on compressivespring 41.

[0264] Other aspects of the configuration and operation of this engineare identical to corresponding aspects of the second embodiment, andhave been given the same numbers.

[0265]FIGS. 12 through 18 show the fourth preferred embodiment of thisinvention. In this embodiment, the respective supply systems for thepreceding air and the fuel mixture have been improved. The basicconfiguration of this engine is identical to that of the first throughthird embodiments. Corresponding parts have been given the same numbers.

[0266] In FIGS. 12 through 18, 41 is an insulator gasket, which isplaced between the surfaces where the insulator 30 and cylinder 2 aremounted to each other; 43 is a carburetor gasket, which is placedbetween the surfaces where the insulator 30 and carburetor 12 aremounted to each other.

[0267] The carburetor gasket 42, which is shown in FIG. 14, has twoparallel passages running through it, air passage 10 b in its upperportion and fuel-mixture passage 15 in its lower portion, in the samepositions as in the insulator 30. Connecting passage 43 is a slit whichconnects the air passage 10 b and fuel-mixture passage 15. 46 are boltholes.

[0268] The insulator gasket 41, which is shown in FIG. 16, has the sametwo parallel passages running through it, air passage 10 b in its upperportion and fuel-mixture passage 15 in its lower portion, at the samepitch as in the insulator 30. Connecting passage 44 is a slit which runsbetween and connects the air passage 10 b and fuel-mixture passage 15.61 is a space for the non-return valve 16; 49 are bolt holes.

[0269] Connecting passage 44 in the insulator gasket 41, as can be seenin FIG. 15, connects fuel-mixture passage 15 and air supply chamber 10in a location which is downstream from flat surface 45 of the non-returnvalve 16. Its diameter, that is, the diameter of its passage, is smallerthan that of connecting passage 43 in carburetor gasket 42. Creating theconnecting passage 44 minimizes any drops in output.

[0270] Instead of providing connecting passages 44 and 43 in theinsulator gasket 41 and carburetor gasket 42, a connecting passage 47can be cut into the surface of insulator 30 which comes in contact withcarburetor gasket 42, as shown in FIG. 14; or a connecting passage 51can be cut into the surface of the insulator 30 which comes in contactwith insulator gasket 41, as shown in FIG. 17.

[0271] The connecting passages may also be cut into the surface ofcylinder 2 where it is attached to the insulator 30, or into the surfaceof carburetor 12 where it is attached to the insulator 30.

[0272] When a preceding air layer-type scavenging two-stroke engineconfigured in this way operates, the combustion pressure in combustionchamber 25 forces piston 4 downward and exhaust port 13 a opens. Thecombustion gases (i.e., the exhaust gas) in the combustion chamber 25exit through exhaust port 13 a into exhaust pipe 13. They pass throughthe muffler (not pictured) and are released into the atmosphere.

[0273] As piston 4 goes further down, scavenging ports 9 a to its leftand right open, and the air stored in scavenging ports 109 e andelsewhere flows into combustion chamber 25, pushing the combustion gasestoward exhaust port 13 a.

[0274] Next, the fuel-air mixture stored in crank chamber 5 a flows intocombustion chamber 25 through scavenging ports 9 a by way of scavengerpassage outlets 109 b and scavenger passages 109 d and 109 e.

[0275] When piston 4 is at bottom dead center, exhaust port 13 a andscavenging ports 9 a open. The supplying of air and fuel-air mixture tocombustion chamber 25 is now complete or on the verge of completion.When piston 4 rises from bottom dead center, it causes scavenging port 9a to close, creating a closed space in crank chamber 5 a. The expansionprocess, that is to say, the depressurization, now begins.

[0276] As piston 4 rises further, exhaust port 13 a closes and themixture of fuel and gas in combustion chamber 25 begins to bepressurized. When piston 4 goes up, the volume of crank chamber 5 aincreases, causing the pressure in the crankcase to decrease. Whenpiston 4 rises still further, air intake port 15 a on the side ofcylinder 2 opens. The fuel-air mixture generated in carburetor 12, whoserate of flow is controlled by valve 14, is supplied via fuel passage 15to crank chamber 5 a.

[0277] The pressure inside the crank chamber 5 a drops, and the lowerpressure is communicated via outlets 109 band scavenger passages 109 dand 109 e to branching air passages 10 a on the left and right.Reed-type non-return valve 16 opens, and the air supplied to air supplychamber 10 via the valve 16 flows into crank chamber 5 a.

[0278] The various pairs of passages which run from the scavenging ports9 a to crank chamber 5 a, namely, branching scavenger passages 109 e andscavenger passages 109 d, together form two long scavenger passages. Theair supplied to the scavenger passages must fill their entire lengthbefore it is admitted into crank chamber 5 a.

[0279] When piston 4 reaches the vicinity of top dead center, spark plug8 discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

[0280] When piston 4 goes down and exhaust port 13 a opens, thecombustion gases in combustion chamber 25 flow through exhaust port 13 ainto exhaust pipe 13. They are exhausted to the exterior through themuffler (not pictured).

[0281] When piston 4 begins to drop, the gases in crank chamber 5 a arepressurized by its reverse side. When piston 4 drops further, theoutlets of scavenging ports 9 a on either side of it open. The fuel-airmixture supplied to crank chamber 5 a as described above is sucked intocombustion chamber 25 from scavenging ports 9 a via outlets 109 b andscavenger passages 109 d and 109 e. The combustion gases (i.e., theexhaust gas) in the combustion chamber 25 are pushed out through exhaustport 13 a and the scavenging operation commences.

[0282] When the engine is idling, the negative pressure in scavengerpassages 10 b, air supply chamber 10 and branching air passages 10 abecomes greater than in fuel passage 15.

[0283] In the fourth embodiment, the air passage 10 b and fuel-mixturepassage 15 are connected either by small-diameter connecting passages 43and 44 in insulator gasket 41 and carburetor gasket 42 or by slits 47and 51, which are cut into insulator 30. When the engine is idling,then, the fuel-air mixture in fuel passage 15 goes through eithersmall-diameter connecting passages 43 and 44 or slits 47 and 51 into airpassage 10 b and air supply chamber 10.

[0284] Thus when there is too much air in air passage 10 b during suddenacceleration, fuel-air mixture can be added to the air flowing throughair passage 10 b. This prevents the new air being supplied to combustionchamber 25 via scavenging port 9 a from causing there to be excess airin the chamber. It thus also prevents the fuel concentration frombecoming too thin during sudden acceleration and so improves theengine's acceleration characteristics.

[0285] When the engine is running at a high speed, the throttle isopened more, and the pressure differential among fuel passage 15, airpassage 10 b and air supply chamber 10 is virtually eliminated. Thusvirtually no air flows from fuel passage 15 through the small-diameterconnecting passages 43 and 44 or 47 and 51 into either air passage 10 bor air supply chamber 10. This design, then, prevents fuel-air mixturefrom contaminating the scavenging air, which insures that the requiredexhaust specifications can be maintained.

[0286] When the engine is mounted obliquely, the fuel-air mixture infuel passage 15 will flow downstream from the non-return valve 16. Itwill thus flow through the connecting passages on the side of combustionchamber 25, namely passages 44 or 51 in insulator gasket 41 of insulator30, and into air supply chamber 10. This prevents fuel from collectingdownstream from fuel passage 15 and being sucked into the cylindersuddenly when the position of the engine changes, so it eliminatesimperfect combustion due to an excess of fuel.

[0287] To prevent defective operation when the engine is mountedobliquely, as described above, the following must be considered. If theconnecting passage 44 or 51 is provided between air supply chamber 10and fuel passage 15, which are downstream from the non-return valve 16,it is connected to scavenger passage 109 e, which is downstream from thevalve 16. Thus if the diameter of the connecting passage is increased,the output of the engine will drop; however, if the connecting passage43 or 47 is located in carburetor gasket 42 or insulator gasket 41,which are upstream from the non-return valve, the diameter of theconnecting passage can be increased without producing a drop in engineoutput. This further improves the acceleration characteristics duringsudden acceleration.

[0288] With the fourth embodiment, then, small-diameter connectingpassages 43 and 44 are provided between fuel passage 15, air passage 10b and air supply chamber 10, so that negative pressure in the airpassage 10 b and air supply chamber 10 will cause the fuel-air mixturein the fuel passage 15 to flow into the air passage. This obviates theneed for a complicated control device and allows us to achieve theeffect described above through the use of a very simple device.

[0289] If one wishes to provide the connecting passage 43 or 44 ininsulator gasket 41 or carburetor gasket 42, it will be possible tofashion a connecting passage merely by creating a slit of the samediameter as the connecting passage 43 or 44 in either the insulatorgasket 41 or the carburetor gasket 42. Creating a connecting passage inthis way is straightforward and does not require a large number ofprocesses.

[0290] In FIG. 18, which shows another embodiment of the connectingpassage, 63 is a connecting passage consisting of a small hole whichconnects air passage 10 b and fuel passage 15 in insulator 30. 64 is thenon-return valve on the connecting passage 63 which permits flow fromthe fuel passage only in the direction toward air passage 10 b.

[0291] The non-return valve 64 may be eliminated, if desired, andconnecting passage 63 may consist only of the small hole. Alternatively,a connecting passage 63 and non-return valve 64 as described above maybe provided in carburetor 12 or cylinder 2 instead of in the insulator30.

[0292] In FIGS. 19 through 22, which show the fifth preferred embodimentof this invention, 1 is the engine, 2 is the cylinder of the engine and5 is the crankcase. The cylinder 2 and crankcase 5 are fastened togetherat surfaces 04 and 05 by a number of bolts 110, with gasket 311 placedbetween them. 13 a is the exhaust port.

[0293]9 a is the scavenging port, which opens into the side of theaforesaid cylinder 2. 209 a is the scavenging passage in cylinder 2,which is connected to the aforesaid scavenging port 9 a. 209 c is theoutlet of the scavenging passage which opens into crank chamber 5 a inthe aforesaid crankcase 5. 209 b is the scavenging passage in theaforesaid crankcase 5. With the help of surface 511 of guide 501, whichwill be discussed shortly, it forms a smoothly curved passage inside thecrankcase 5. Scavenging passage 209 a in the aforesaid cylinder 2 andscavenging passage inlet 209 c are connected at the aforesaid surfaces04 and 05 where the passages in crankcase 5 and cylinder 2 meet.

[0294]1001 is the air passage for preceding air. It is connected partwayalong the length of the aforesaid scavenging passage 209 a. Precedingair from the air cleaner (not pictured) is supplied to scavenging port 9a through the air passage and scavenging passage 209 a.

[0295] With the exception of guide 501, the configuration just describedis identical to the prior art design shown in FIG. 39.

[0296]501 is a guide. It is inserted into the aforesaid crankcase 5 fromsurface 04 to provide surface 511, which is smoothly connected toscavenging passage 209 b in the crankcase.

[0297] As can be seen in FIGS. 21 and 22, the guide 501 has acylindrical protrusion 531 on its upper surface. Two teeth, 541 and 551,protrude on its sides. As can be seen in FIG. 20, when the teeth 541 and551 on the guide 501 engage in depressions 151 and 141 in crankcase 5,the guide is fixed to the crankcase. If one set of teeth anddepressions, for example tooth 541 and depression 151, is engaged moreloosely than the other, in this case tooth 551 and depression 141, guide501 can easily be installed.

[0298] Projection 531 on the guide 501 engages in the hole (notpictured) in the aforesaid gasket 311. Thus when gasket 311 is installedon top of guide 501, the correct placement of the gasket 311 can bechecked by verifying the position of projection 531. Also, the fact thatthe guide 501 has been installed can be confirmed by looking at gasket311, so there is no chance of forgetting to install the guide 501.

[0299] There is a depression 521 on the upper surface of the aforesaidguide 501. When fuel from the fuel-air mixture gets into the slight gapwhere gasket 311 separates the crankcase from the cylinder, the fuelwill flow downward through the depression 521.

[0300] In a two-stroke engine with a scavenging passage configured inthis way, the fuel-air mixture from crank chamber 5 a in crankcase 5 isconducted into scavenging passage 209 b, a portion of which consists ofthe smoothly curved surface 511 of guide 501. It flows through thesmoothly curved scavenging passage 209 b and is supplied to scavengingport 9 a.

[0301] Because scavenging passage 209 b is a smoothly curved channelwithout any right angles, the fuel-air mixture flows through it smoothlyand rapidly without any flow loss such as a decrease in flow velocity asit is supplied to scavenging port 9 a.

[0302] With the fifth preferred embodiment, the aforesaid guide 501 isinstalled in crankcase 5 in such a way that it can be attached orremoved from surface 04 of cylinder 2 in the axial direction 50 of thecylinder. This obviates the need for the tooth 161 between the upperwall of scavenging passage 209 b and the surface 04 to which thecrankcase is attached, as was required in the prior art. The guide 501performs the same function as the aforesaid tooth 161. Thus whencrankcase 5 is cast, even if a single die is used to form scavengingpassage 209 b inside the crankcase 5, the die can easily be removed inthe axial direction 50 of the cylinder. Because the scavenging passagecan be formed using a single die, there is no possibility that one ofseveral dies will slip out of position, as sometimes happened with priorart techniques, and ruin the casting.

[0303] Also, with the fifth preferred embodiment, if the fuel in thefuel-air mixture flowing through scavenging passages 209 a and 209 b,which are connected where they meet between surfaces 04 and 05 ofcrankcase 5 and cylinder 2, seeps into the gap between surfaces 04 and05 where gasket 311 is inserted, the fuel will flow downward through thedepression 521 formed in guide 501. Further, creating the aforesaiddepression 521 reduces the surface area of the aforesaid gap, so lessfuel will seep into the gap. Thus even if the engine is mountedobliquely, the fuel cannot return to scavenging passages 209 a and 209b. This prevents the imperfect combustion which would result if fuelcould flow back into scavenging passages 209 a and 209 b.

[0304] In addition to the aforesaid embodiments, the following twomodifications are also included in the scope of this invention.

[0305] In the first modification, the aforesaid guide 501 consists of aflat piece which is formed integrally to gasket 311 from a resin. Thepiece corresponding to the guide is formed by molding a deep-drawingsheet. This surface 511, which connects smoothly with scavenging passage209 b in the aforesaid crankcase 5, and it also forms depression 521 onthe reverse side of the surface.

[0306] With this modification, guide 501 and gasket 311 are one piece,and deep drawing allows channel surface 511 and depression 521 to beformed on the front and reverse sides of the sheet at the same time.This reduces the parts count and the number of assembly processes.

[0307] In the second modification, the aforesaid guide 501 is given adifferent color than the rest of the crankcase assembly. If the guide isa different color, its presence or absence will be all the more evident,so it will be impossible to forget to install it. By using a differentcolor for each type of machine, we can simplify our parts control.

[0308]FIG. 27 shows an air-layer-type scavenging two-stroke engine inwhich the sixth and seventh preferred embodiments of this invention havebeen implemented. In this figure, 2 is the cylinder; 421 is the interiorwall of the cylinder; 4 is the piston; 6 is the crankshaft; 6 a is thecrank web, a constituent of the crankshaft 6; 5 is the crankcase; 3 isthe connecting rod which links piston 4 to crankshaft 6; 7 is thecylinder head; 8 is the spark plug; 11 is the air cleaner; and 12 is thecarburetor.

[0309]25 is the combustion chamber; 5 a is the crank chamber insidecrankcase 5; and 15 is the fuel passage which connects the aforesaidcarburetor 12 to crank chamber 5 a. 13 a is the exhaust port on the sideof cylinder 2. It is connected to the exhaust pipe by exhaust passage411.

[0310]9 a are the two scavenging ports, which face each other oncylinder 2 to the right and left of exhaust port 13 a at virtually aright angle with respect to the exhaust port. The scavenging ports 9 acommunicate with the aforesaid crank chamber 5 a via branchingscavenging passages 109 e, which are angled obliquely with respect tocylinder 2; arc-shaped scavenging passages 109 d, which are formedinside the walls on either side of crankcase 5; and outlets 109 b.

[0311]10 is the air supply chamber formed on the side of cylinder 2. Itsupstream side is connected to air passage 10 b in insulator 30; itsdownstream side is connected to branching air passages 10 a. Thebranching air passages 10 a connect to the two branching scavengingpassages 109 e.

[0312] Non-return valve 16 on the outlet of the air supply chamber 10,which goes to branching air passages 10 a on the right and left, permitsair to flow only in the direction of the air passages.

[0313] The aforesaid insulator 30 thermally isolates the air intakesystem from the engine body. It is bolted to the side of cylinder 2. Theaforesaid air passage 10 b is in the upper portion of the insulator 30and fuel passage 15 is in the lower portion.

[0314] The upstream side of the fuel passage 15 is connected to valve 14on carburetor 12, which controls the flow rate of the fuel-air mixture.The downstream side is connected to the inside of the cylinder (i.e., tocombustion chamber 25) via air intake port 15 a.

[0315]110 is an air passage integral to carburetor 12 which connects aircleaner 11 to insulator 30. On the air passage 110 is an air controlvalve 20, which changes the diameter of the passage. The air controlvalve 20 is interlocked with mixture control valve 14 on carburetor 12.

[0316] This invention concerns an improvement in the configuration ofthe scavenging passages in the air layer-type scavenging two-strokeengine discussed above.

[0317]FIGS. 23 and 25 show the sixth preferred embodiment of thisinvention. 2 is the cylinder; 25 is the combustion chamber inside thecylinder; 431 is the space for the aforesaid spark plug 8; 15 is thefuel passage; 15 a is the air intake port which is the outlet of thefuel passage 15 into combustion chamber 25; 13 a is the exhaust port;and 411 is the exhaust passage which connects the chamber to the exhaustport 13 a. 421 is the interior wall of the cylinder.

[0318]9 a is one of the scavenging ports. It is actually the outlet ofone of the aforesaid scavenging passages 109 d into combustion chamber25. As in FIG. 27, there are two such passages and outlets.

[0319] As is shown in FIG. 25, upper walls 9 c and 9 b of the aforesaidscavenging passages 109 d, which connect to the aforesaid scavengingports 9 a, and their blow-up angle α, the angle they form with respectto a passage perpendicular to the axis 50 of the cylinder, or in otherwords with respect to horizontal passage 45, vary along the periphery ofthe cylinder.

[0320] The sixth preferred embodiment is shown in FIG. 23. If we callthe blow-up angle of the aforesaid scavenging passage 109 d, which canbe seen in FIG. 25, in a location nearer the aforesaid exhaust port α1(B in FIG. 25) and that in a location nearer the aforesaid intake portα2 (A in FIG. 25), then α1<α2. The blow-up angle α varies continuouslyfrom a location nearer intake port 15 a (α2) to one nearer exhaust port13 (α1).

[0321] In the seventh embodiment of this invention, which is pictured inFIGS. 24, 26 and 27, surfaces 9 c and 9 b of the upper wall of theaforesaid scavenging passage 109 d are formed so that they vary in stepfashion from blow-up angle α2 along a given length a on the side of thepassage nearer intake port 15 a to blow-up angle α1 along a given lengthb on the side of the passage nearer exhaust port 13, with the angulardifference mediated by step 441.

[0322] In this case there is a fixed blow-up angle α2 along the entirelength of the aforesaid given length a, and a different fixed blow-upangle α1 along the entire length of the aforesaid given length b. Itwould also be permissible to have two or more steps like the aforesaid441 so that the blow-up angle would vary in three or more stages.

[0323] When an air layer-type scavenging two-stroke engine configured inthis way operates, the combustion pressure in combustion chamber 25forces piston 4 downward and opens exhaust port 13. The combustion gases(i.e., the exhaust gas) in the combustion chamber 25 flow throughexhaust port 13 and exhaust passage 411 to the exhaust pipe, and arereleased into the atmosphere through the muffler (not pictured).

[0324] When piston 4 goes down further, the scavenging ports 9 a to itsleft and right open. The air which has accumulated in branchingscavenging passages 109 e flows into combustion chamber 25, pushing thecombustion gases toward exhaust port 13.

[0325] Next, the fuel-air mixture stored in crank chamber 5 a flows intocombustion chamber 25 via outlets 109 b of the scavenging passages,scavenging passages 109 d and branching scavenging passages 109 e.

[0326] When piston 4 reaches bottom dead center, exhaust port 13 andscavenging ports 9 a open, and the supply of air and fuel-air mixture tocombustion chamber 25 is completed or virtually completed. Scavengingports 9 a are closed by the action of the piston 4, and the interior ofcrank chamber 5 a becomes a closed space. As the space begins to expand,its pressure begins to drop.

[0327] When piston 4 rises further, exhaust port 13 closes, and thefuel-air mixture in combustion chamber 25 begins to be pressurized. Aspiston 4 rises, the volume inside crank chamber 5 a increases, whichfurther reduces the pressure in the crank chamber 5 a. When piston 4rises further, air intake port 15 a on the side of cylinder 2 opens, andthe fuel-air mixture generated in carburetor 12, and whose flow rate iscontrolled by valve 14, is supplied to crank chamber 5 a through fuelpassage 15.

[0328] The drop in pressure inside the aforesaid crank chamber 5 a iscommunicated via outlets 109 b, scavenging passages 109 d and branchingscavenging passages 109 e to branching air passages 10 a on the left andright. Non-return valve 16 opens, and the air supplied to air supplychamber 10 fills scavenging passages 109 d.

[0329] When piston 4 reaches the vicinity of top dead center, spark plug8 discharges a spark in combustion chamber 25. This ignites thepressurized fuel-air mixture and combustion occurs. The pressuregenerated by this combustion pushes piston 4 down, which generatesrotary torque in crankshaft 6.

[0330] When piston 4 goes down and exhaust port 13 opens, the combustiongases in combustion chamber 25 flow through exhaust port 13 into theexhaust pipe. They are exhausted to the exterior through the muffler(not pictured).

[0331] In the aforesaid scavenging operation, because the blow-up angleα2 of upper walls 9 c and 9 b of the aforesaid scavenging passage 109 dis greater at a location nearer intake port 15 a than blow-up angle α1at a location nearer exhaust port 13 a, the fuel mixture which entersthe chamber from the location nearer the exhaust port 13 a will flowalong the top of the piston at a high speed without being dispersed.This will prevent it from getting caught in the exhaust gas stream andso reduce the quantity of fuel lost through exhaust port 13 a.

[0332] The fuel-air mixture which enters the chamber from the locationnearer the aforesaid intake port 15 of the aforesaid scavenging passage109 d will be flowing at a lower velocity than that nearer the aforesaidexhaust port 13 a. It will be sent into the area around the spark plugin the upper part of the chamber, where it will be efficiently ignitedand combusted.

[0333] Thus the sixth and seventh embodiments prevent the fuel-airmixture supplied to combustion chamber 25 from escaping unburned throughexhaust port 13 a, improve the scavenging efficiency, and increase theconcentration of the fuel-air mixture which fills combustion chamber 25.

[0334] If the engine is configured as in the aforesaid seventhembodiment, the surfaces 9 c and 9 b of the upper wall of the aforesaidscavenging passage 109 d is formed so that it varies in step fashionfrom a large blow-up angle α2 at a location nearer intake port 15 a to asmaller angle α1 at a location nearer exhaust port 13 a, with the changein angles occurring at step 441. When the cylinder is cast, two dies canbe used with two different blow-up angles α, as described above, withthe angles changing at the step 441. This will make it easy to removethe work from the dies.

[0335] Also, using dies with two different blow-up angles to formscavenging passage 109 d is an easy and reliable way to control theblow-up angle.

[0336]FIGS. 28 through 31 show the eighth preferred embodiment of thisinvention. In FIG. 31, 2 is the cylinder, 4 the piston, 6 thecrankshaft, 6 a the crank web of the crankshaft 6, 5 the crankcase, 3the connecting rod which connects piston 4 and crankshaft 6, 7 thecylinder head, 8 the spark plug, 11 the air cleaner and 12 thecarburetor. 25 is the combustion chamber. 5 a is the crank chamberformed inside crankcase 5. 15 is the passage for the fuel-air mixturewhich connects the aforesaid carburetor 12 and crank chamber 5 a. 13 ais the exhaust port on the side of cylinder 2. It is connected toexhaust passage 411.

[0337]9 a are the two scavenging ports which face each other on cylinder2 to the right and left of exhaust port 13 a at virtually a right anglewith respect to the exhaust port. As can be seen in FIG. 30, thescavenging ports 9 a communicate with the aforesaid crank chamber 5 avia branching scavenging passages 109 e, which are angled obliquely withrespect to cylinder 2; scavenging passages 109 f, which are at thepoints where the various scavenging passages meet; arc-shaped scavengingpassages 109 d, which are formed inside the walls on either side ofcrankcase 5; and outlets 109 b.

[0338] The end surfaces of the aforesaid outlets 109 b and the endsurfaces of crank webs 6 a approximate each other in the direction ofcrankshaft 60, leaving only a microscopic gap, so that the ends of theoutlets can be opened and closed by the revolution of crank webs 6 a ofcrankshaft 6.

[0339]10 is the air supply chamber formed on the side of cylinder 2. Itsupstream side is connected to air passage 10 b in insulator 30, whichwill be discussed shortly; its downstream side is connected to branchingair passages 10 a. As can be seen in FIG. 30, the branching air passages10 a connect to scavenging passages 109 f and 109 e.

[0340] Non-return valve 16 on the outlet of the air supply chamber 10,which goes to branching air passages 10 a on the right and left, permitsair to flow only in the direction of the air passages.

[0341] As is shown in FIG. 30, the aforesaid branching air passages 10 aand scavenging passages 109 e are formed virtually symmetrically withrespect to the axis 50 of the cylinder by walls 109 h and 109 i, whichextend outward from the sides of the cylinder 2 and are integral to it.The walls 109 h and 109 i are parallel to each other. The single diewhich forms them can be removed by pulling it sideward away from thecylinder.

[0342]30 is an insulator to thermally isolate the engine body from theair intake system. The insulator 30 is bolted to the side of cylinder 2.The aforesaid air passage 10 b is in the upper portion of the insulator30, and fuel passage 15 is in its lower portion. The upstream side ofthe fuel passage 15 is connected to carburetor 12, as described above.The downstream side is connected to the interior of the cylinder(combustion chamber 25) via air inlet port 15 a.

[0343] In FIGS. 28 and 29, the aforesaid crankcase 5 consists of frontportion 05 a and rear portion 05 b. The two portions of the crankcaseare divided at surface 512, a surface at a right angle with respect tocrankshaft 60 and cylinder axis 50, which is also the axis of thecrankshaft. After crankshaft 6 and main shaft bearings 522 are installedbetween them, the front portion 05 a and rear portion 05 b are fastenedto each other by a number of bolts 542.

[0344]533 is the flat surface on the top of the aforesaid crankcase 5 towhich the cylinder will be mounted. The undersurface 532 of cylinder 2is brought up against the surface 533 and the cylinder is fastened tothe aforesaid crankcase 5 by a number of bolts 552.

[0345] Inside the aforesaid front portion 05 a and rear portion 05 b aretwo symmetrical scavenging passages 109 d and their outlets 109 b. Thetwo passages meet where they go through common surface 512. The upperends of the scavenging passages 109 d come out through the surface 533,so the upper portions of the dies which form them can be removed bypulling them away from the surface 533.

[0346] Inside the aforesaid cylinder 2, scavenging passages 109 f, whichconnect with scavenging passages 109 d in the aforesaid front portion 05a and rear portion 05 b of the crankcase; branching scavenging passages109 e, which connect with the scavenging passages 109 f; and branchingair passages 10 a, which connect with the branching scavenging passages109 e, are symmetric with respect to surface 512 of the aforesaidcrankcase 5.

[0347] When an air layer-type scavenging two-stroke engine configured inthis way operates, piston 4 goes down and the scavenging ports 9 a toits left and right open. The air which has accumulated in branchingscavenging passages 109 e by flowing through the aforesaid air passage10 b, non-return valve 16 and air supply chamber 10 flows throughscavenging ports 9 a into combustion chamber 25 and pushes thecombustion gases toward exhaust port 13 a.

[0348] Next, the fuel-air mixture stored in crank chamber 5 a flowsthrough scavenging ports 9 a into combustion chamber 25 via scavengingpassage outlets 109 b, scavenging passages 109 d and 109 f and branchingscavenging passages 109 e.

[0349] When piston 4 rises from bottom dead center, it closes scavengingports 9 a, and the interior of crank chamber 5 a becomes a closed space.As the space begins to expand, its pressure begins to drop.

[0350] When piston 4 rises further, exhaust port 13 a closes, and thefuel-air mixture in combustion chamber 25 begins to be pressurized. Aspiston 4 rises, the volume inside crank chamber 5 a increases, whichfurther reduces the pressure in the crankcase. When piston 4 risesfurther, air intake port 15 a opens, and the fuel-air mixture issupplied to crank chamber 5 a through fuel passage 15.

[0351] When an air layer-type scavenging two-stroke engine configured asdescribed above is manufactured, scavenging passages 109 d are castinside front portion 05 a and rear portion 05 b of the crankcase, andthe required maching processes are performed. Then piston 4, crankshaft6, connecting rod 3 and main shaft bearings 522 are assembled betweenthe two halves of the crankcase. The halves are joined at surface 512and fastened together by bolts 542 to achieve a unitary crankcase 5.

[0352] In cylinder 2, scavenging passages 109 f, which connect withpassages 109 d in the aforesaid crankcase 5; branching scavengingpassages 109 e, which connect with the passages 109 f; and branching airpassages 10 a, which connect with the branching passages 109 e, are castin such a way that they are symmetric with respect to surface 512 of theaforesaid crankcase 5, and the required machining processes areperformed.

[0353] Piston 4, connecting rod 3 and other necessary components areassembled in the aforesaid cylinder 2, and its undersurface 532 isbrought up against surface 533 of the aforesaid crankcase 5. Thecylinder is then fastened to the crankcase by bolts 552.

[0354] With the eighth embodiment, then, crankcase 5 is divided atsurface 512, a surface at a right angle with respect to crankshaft 60and cylinder axis 50, which is also the axis of the crankshaft, into afront portion 05 a and a rear portion 05 b, both of which havescavenging passages 109 d running inside them. The front and rearportions 05 a and 05 b are fastened together by bolts 542. Cylinder 2,which contains scavenging passages 109 f, branching scavenging passages109 e and branching air passages 10 a, is fastened to surface 533 on thetop of the aforesaid crankcase 5 by bolts 552. Scavenging passages 109 fon the bottom of the cylinder communicate with scavenging passages 109 din crankcase 5, forming two long scavenging passages which run throughboth crankcase 5 and cylinder 2. The crankcase 5 and cylinder 2 thusassume a compact shape with no projections, and their scavengingpassages are smooth passages with no sharp angles.

[0355] The engine is divided into front and rear portions 05 a and 05 b,with scavenging passages 109 d running through both portions, and thetwo portions are cast separately. Thus the work can be removed from thedies at the aforesaid surface 512 where the two portions will be joinedor at surface 533, the surface perpendicular to surface 512 where thecylinder is mounted. This simplifies the shapes of the dies and theremoval of the work and reduces the number of dies needed.

[0356] With the eighth embodiment, scavenging passages 109 d and 109 f,branching scavenging passages 109 e and branching air passages 10 a areall symmetric with respect to surface 512 of the aforesaid crankcasealong their entire length from outlets 109 b into crank chamber 5 a toscavenging ports 9 a in cylinder 2. A common die can therefore be usedto cast the two respective scavenging passages and branching airpassages in the front and rear portions of the crankcase, so fewer diesare needed. The shape described above makes the passages of the twosymmetric scavenging passages exactly the same size. Cylinder 2 willtherefore be filled uniformly along its circumference with scavengingair and fuel-air mixture. And because the aforesaid two branching airpassages 10 a also have identically-shaped passages, cylinder 2 will bescavenged uniformly along its circumference.

[0357] Walls 109 h and 109 i of the aforesaid scavenging and branchingair passages can be formed integrally to cylinder 2 and virtuallyparallel with each other. Because this design allows the cylinder to becast using a single sliding die, it simplifies the configuration of thedie and reduces the cost of producing it.

[0358]FIGS. 32 through 38 show the ninth preferred embodiment of thisinvention. In FIG. 32, 2 is the cylinder; 5 is the crankcase. Thecylinder 2 and crankcase 5 are fastened to each other with gasket 05between them by bolts 110 at their mounting surfaces. 4 is the piston; 3is the connecting rod; 8 is the spark plug; 13 a is the exhaust port;and 25 is the combustion chamber.

[0359]9 a is the scavenging port, which is on the side of the aforesaidcylinder 2. 5 a is the crank chamber inside the aforesaid crankcase 5.12 is the carburetor; 30 is the insulator between the carburetor 12 andthe aforesaid cylinder 2. 15 is the passage for the fuel-air mixture. Itruns from the throttle passage of the aforesaid carburetor 12 throughinsulator 30 and cylinder 2 to the aforesaid crank chamber 5 a. 10 b isthe air passage. It runs from the air passage of the aforesaidcarburetor 12 through insulator 30 and non-return valve 16 to thescavenging passage and the aforesaid scavenging port 9. The non-returnvalve 16 is a reed valve which is opened and closed by negative pressurein the aforesaid scavenging passage.

[0360] In the two-cycle engine according to this invention, the aircleaner and the choke valve used in it have been improved.

[0361]1001 is the air cleaner, which is configured as follows.

[0362]1011 is the air cleaner housing. It is attached to the aforesaidcarburetor 12 by bolts (not pictured). 1021 is the air cleaner cover. Itis attached to the aforesaid air cleaner housing 1011 by bolts 109.

[0363] Two air passages, passage 1061 and passage 1071, run parallel toeach other through the aforesaid air cleaner housing 1011. Air passage1061 connects to air passage 10 b, which has the aforesaid non-returnvalve 16 on it. Air passage 1071 connects to the aforesaid fuel passage15.

[0364]1201 is a choke valve which alternately opens and closes theaforesaid air passages 1061 and 1071.

[0365] In FIGS. 32 through 38, 1041 is a rotary valve consisting ofvalve portions 1041 a and 1041 b. When rotated, it engages over theopening in the center of the aforesaid air cleaner cover 1021. Valveportions 1041 a and 1041 b alternately open and close the inlets of theaforesaid air passages 1061 and 1071. 1031 is the rotary knob whichoperates the valve 1041. It is fixed to the end of the valve 1041. Thereis a choke hole 1501 in valve portion 1041 b. On the side of valveportions 1041 a and 1041 b which is away from the engine extensions 1511and 1521 are formed integrally to the valve portions.

[0366] The end of the aforesaid valve 1041 consists of flat surface1101, which covers or uncovers the inlets of the aforesaid air passages1061 and 1071. Flat surface 1101 of the valve 1041, as can be seen inFIG. 32, is shaped in such a way as to completely obstruct the inlets ofthe aforesaid air passages 1061 and 1071 when the valve 1041 is rotated.

[0367]1051 is an O-ring which is placed on the circumference of thevalve stem of the aforesaid valve 1041. It goes between the innersurface 1131 of the aforesaid air cleaner cover 1021 and step portion1221 of the valve stem of the aforesaid valve 1041 and exerts pressurein the axial direction of the valve, forming a liquid seal for theinterior of air cleaner 1001. Its elastic force presses flat surface1101 of the aforesaid valve 1041 against the inlets of the aforesaid airpassages 1061 and 1071.

[0368] As FIG. 33 shows, when rotary knob 1031, which is integral to theaforesaid valve 1041, is turned approximately 90° clockwise fromstarting position, it is set in normal operating position. 1231 is astop which projects from the exterior surface of the aforesaid aircleaner cover 1021. As can be seen in FIGS. 33 and 34, when theaforesaid rotary knob 1031 is being rotated from the aforesaid startingposition to normal operating position, its end goes over the aforesaidprojection 1231 against the force of the aforesaid O-ring 1051.Extensions 1511 and 1521 on valve 1041 come up against and are stoppedby stops 1531 and 1541, as shown in FIG. 37. The elastic force of theaforesaid O-ring 1051 returns the valve to its previous position, and itis held on the portion of the aforesaid air cleaner cover 1021 whichdoes not have a projection 1231 on it.

[0369] As can be seen in FIGS. 37 and 38, there is a projection 1111with a tapered surface 1121 on the inside of the aforesaid air cleanercover 1021. When the aforesaid rotary knob 1031 is in starting position,as shown in FIG. 33, its end rides up on tapered surface 1121 of theprojection 1111, and flat surface 1101 of the aforesaid valve 1041pushes against the inlet of the aforesaid air passage 1061, closing itcompletely.

[0370] In an air cleaner in a two-stroke engine configured as describedabove, when the engine is going to be started up, the rotary knob 1031of choke valve 1201 is set in starting position (choke position), asshown in FIG. 33. When rotary valve 1041, which is fixed to the knob1031, is rotated, the air inlet of carburetor 12 and the aforesaid airpassage 1071, which is connected to the aforesaid fuel passage 15, isfully closed (although the small choke hole 1501 remains open). Theaforesaid air passage 1061, which is connected to the air passage 10 bwhich supplies preceding air, is also fully closed, and the engine isstarted up.

[0371] When this choke valve 1201 is operated, the air filtered bycleaner element 1081 of air cleaner 1001 flows into air passage 1071through choke hole 1501 and is supplied from the passage 1071 to themain nozzle of carburetor 12, which is connected to passage 1071. In thecarburetor 12, a fuel-air mixture is generated by atomizing the fuel inthe air. The fuel-air mixture is supplied from fuel passage 15 throughcrank chamber 5 a, the scavenging passage and scavenging port 9 a, intocombustion chamber 25, where it is ignited and combusted, thus startingthe engine.

[0372] When the engine is started up, air passage 1061 of air cleaner1001 is fully closed by valve 1041 of the aforesaid choke valve 1201.Thus preceding air cannot be supplied to the combustion chamber throughthe air passages 1061 and 10 b. Only the fuel-air mixture generated incarburetor 12 using the air which enters the aforesaid air passage 1071through choke hole 1501 is supplied to the combustion chamber. Thuscombustion chamber 25 will be filled with a rich fuel-air mixture, andthe engine's starting characteristics will improve.

[0373] Flat surface 1101 of rotary valve 1041 in the aforesaid chokevalve 1201 opens and closes the inlets of the aforesaid air passages1061 and 1071. 1051 is an O-ring which is placed on the circumference ofthe stem of the aforesaid valve 1041 between the inner surface 1131 ofthe aforesaid air cleaner cover 1021 and step portion 1221 of the stemof the aforesaid valve 1041. It exerts pressure in the axial directionof the valve 1041, forming a liquid seal for the interior of air cleaner1001. Its elastic force presses flat surface 1101 of the aforesaid valve1041 against the inlets of the aforesaid air passages 1061 and 1071.Thus the choke valve 1201 completely closes the inlet of air passage1061 for preceding air. This allows a rich mixture to be generated, asdescribed above, and it allows a high negative pressure to bemaintained.

[0374] In addition to the aforesaid O-ring 1051, a projection 1231 whichserves as a stop is provided on the outer surface of the aforesaid aircleaner cover 1021. When the aforesaid choke valve 1201 is rotated fromstarting position to normal operating position, it switches between theaforesaid air passages 1061 and 1071. When the aforesaid rotary knob1031 is between starting position and normal operating position, its endgoes over the aforesaid projection 1231 against the force of theaforesaid O-ring 1051. The moderate friction improves the operating feelof the choke valve. When the choke valve 1031 is released, the elasticforce of the aforesaid O-ring 1051 and the force of the aforesaidprotruding stop 1231 automatically hold the choke valve 1201 in place onthe flat portion of the outer surface of air cleaner 1021 in such a waythat it cannot go back.

[0375] There is a projection 1111 with a tapered surface 1121 on theinside of the aforesaid air cleaner cover 1021. As can be seen in FIG.33, when the aforesaid choke valve 1201 is rotated toward startingposition, the end of rotary knob 1031 will ride up on tapered surface1121 of the projection 1111 when the knob reaches starting position. Theflat surface 1101 of the aforesaid valve 1041 will then press againstthe inlet of the aforesaid air passage 1061.

[0376] This improves the seal formed by flat surface 1101 of the valve1041 so that the aforesaid air passage 1061 can be completely closedoff.

[0377] A rotary choke valve 1201 configured as described above is notlimited in its application to use as a choke valve for an air cleaner asdescribed above. It can be used for a wide range of applications whichrequire a valve to switch between two fluid passages when knob 1031 isturned to rotate valve 1041.

1. A two-stroke cycle engine using a preceding air-layer for scavenging,comprising: an exhaust port on the side of the cylinder; a scavengingport on the side of the cylinder; a fuel passage, which suppliesfuel-air mixture to the crank chamber through the intake port on theside of the cylinder during the time of the elevation of the piston; anair passage, which supplies scavenging air from the air cleaner towardsthe inner side of said engine; an insulator, in which said fuel passageand said air passage run in parallel; a non-return valve, which isprovided on said insulator facing towards the inner side of said engine,to allow the scavenging air to flow only towards the inner side of saidengine; a pair of branching air passages to connect an air supplychamber provided at the inner side of said non-return valve and abranching scavenger passage opened to said scavenging port, which areprovided within the wall of the cylinder: and a pair of scavengerpassages, one end of which is connected to said scavenging port, andanother outlet end of which is opened to the crank chamber, said pair ofscavenger passages are provided within the wall of the crankcase.
 2. Atwo-stroke cycle engine using a preceding air-layer for scavengingaccording to claim 1, wherein the end surface of said outlet end of saidscavenger passage in the crankcase forms right angles with respect tothe axis of the crankshaft, and a microscopic gap is created betweensaid end surface of said outlet and the end surfaces of the crank webswhich are perpendicular to the crankshaft, which constitutes diskvalves, as the opening area of said outlet of said scavenger passagevaries as the crank webs rotate.
 3. A two-stroke cycle engine using apreceding air-layer for scavenging according to claim 2, wherein saidopening area of said outlet of said scavenger passage is formed so thatsaid opening area opens more with the rotation of the crank webs as saidopening area uncovered by the crank web grows larger.
 4. A two-strokecycle engine using a preceding air-layer for scavenging according toclaim 1, wherein said branching air passages and said branchingscavenger passages formed on either side of the cylinder are surroundedby virtually parallel walls which run in the same direction.
 5. Atwo-stroke cycle engine using a preceding air-layer for scavenging,comprising: an exhaust port on the side of the cylinder; a scavengingport on the side of the cylinder; an intake port on the side of thecylinder fuel passage, which supplies fuel-air mixture through a mixturecontrol valve on the carburetor to the crank chamber during the time ofthe elevation of the piston; a scavenger passage opened to saidscavenging port; an air supply port, which supplies scavenging air fromthe air cleaner to said scavenger passage; a cam which is interlockedwith said mixture control valve; a cam follower which engages with saidcam; and an air control valve in the upstream of an air passage whichcontrols the diameter of said air passage, and said air control valvebeing operated by said cam and said cam follower in such a way as tosupply a quantity of scavenging air proportional to the quantity offuel-air mixture determined by the opening of said cam and the mixturecontrol valve to control the fuel-air mixture.
 6. A two-stroke cycleengine using a preceding air-layer for scavenging according to claim 5,wherein said air control valve comprises a valve seat midway along theair passage and an umbrella-type valve which can be attached to orremoved from said valve seat and which opens and closes said airpassage, said cam is fixed to the rotary shaft of said mixture controlvalve, said cam is configured with an inner cam which is formed on theinside of the edge at a given height raised up on the outer side alongthe circumference so that, if a spring exerts force in the directionwhich closes said air control valve, when the edge of said inner camengages with said cam follower, the operation of said mixture controlvalve for the fuel-air mixture is transmitted to said air control valve,and said operation opens the air control valve against the force of thespring.
 7. A two-stroke cycle engine using a preceding air-layer forscavenging according to claim 5, wherein said air control valvecomprises a valve seat midway along the air passage and an umbrella-typevalve which can be attached to or removed from said valve seat and whichopens and closes said air passage, said cam is fixed to the rotary shaftof said mixture control valve, said cam is configured with an inner camwhich is formed on the inside of the edge at a given height dropped downon the outer side along the circumference so that, if a spring exertsforce in the direction which closes said air control valve, when theedge of said inner cam engages with said cam follower, the operation ofsaid mixture control valve for the fuel-air mixture is transmitted tosaid air control valve, and said operation opens the air control valveagainst the force of the spring.
 8. A two-stroke cycle engine using apreceding air-layer for scavenging according to claim 5, furthercomprising an insulator which is fixed to the side of the cylinder, saidinsulator being provided with said intake port for the fuel-air mixtureand said air supply port for scavenging air provided downstream fromsaid air control valve, both of said ports facing towards the samedirection, and an air supply passage to said intake port facing towardsthe same direction.
 9. A two-stroke cycle engine using a precedingair-layer for scavenging, comprising: an exhaust port on the side of thecylinder; a scavenging port on the side of the cylinder; a fuel passage,which supplies fuel-air mixture to the crank chamber through the intakeport on the side of the cylinder during the time of the elevation of thepiston; a scavenger passage to be connected to said scavenging port; anair passage, which supplies scavenging air from the air cleaner towardthe inner side of said engine; an insulator, in which said fuel passageand said air passage run in parallel; a non-return valve, which isprovided on said insulator facing toward the inner side of said engine,to open or close said air passage by means of the negative pressure insaid scavenger passage; and a connecting passage with a small diameterto link said air passage and said fuel passages so that negativepressure in said air passage forces the fuel-air mixture in said fuelpassage into said air passage.
 10. A two-stroke cycle engine using apreceding air-layer for scavenging according to claim 9, wherein saidconnecting passage links said air passage at a point downstream fromsaid non-return valve and said fuel passage.
 11. A two-stroke cycleengine using a preceding air-layer for scavenging according to claim 9,wherein said connecting passage links said air passage at a pointupstream from said non-return valve and said fuel passage.
 12. Atwo-stroke cycle engine, comprising: a scavenger passage which connectsa scavenging port on the side of the cylinder to the crank chamberinside the crankcase, and goes through the mounting surface where thecylinder and crankcase are attached to each other; and a removable guidewith a surface forming a curved smooth channel which is attachable tosaid scavenger passage in the crankcase from the mounting surface, andforms a portion of said scavenger passage with the curved channel.
 13. Atwo-stroke cycle engine according to claim 12, wherein said removableguide comprises a positioning tooth which engages with the hole in thegasket for the mounting surface where the cylinder and crankcase areattached to each other.
 14. A two-stroke cycle engine according to claim12, wherein said removable guide is fixed to the crankcase when a toothengages in an indentation in the crankcase.
 15. A two-stroke cycleengine according to claim 12, wherein said removable guide has adepression in the mounting surface where the cylinder and crankcase areattached to each other.
 16. A two-stroke cycle engine according to claim12, wherein said removable guide is painted on.
 17. A two-stroke cycleengine, comprising: an exhaust port on the sidewall of the cylinder,which opens into the cylinder; a scavenging port on the sidewall of thecylinder positioned a slight distance apart in the circumferentialdirection from said exhaust port, which also opens into the cylinder; anintake port, which opens to supply fuel-air mixture to the crankcaseaccording to the action of the piston; and a scavenger passage, whichconnects the crankcase and said scavenging port; wherein a blow-up angle(α) of said scavenger passage, which is defined by an angle between theupper wall which connects to said scavenging port and a perpendicularline to the axis of the cylinder, varies along the circumferentialdirection of the cylinder, and if said blow-up angle in a locationnearer said exhaust port is defined as (α1) and said blow-up angle in alocation nearer said intake port is defined as (α2), then α1<α2.
 18. Atwo-stroke cycle engine according to claim 17, wherein said blow-upangle α varies continuously from a location nearer intake port (α2) tosaid blow-up angle nearer exhaust port (α1).
 19. A two-stroke cycleengine according to claim 17, wherein said blow-up angle α varies instep fashion from a location nearer intake port (α2) to said blow-upangle nearer exhaust port (α1).
 20. A two-stroke cycle engine,comprising: a scavenging port on the side of the cylinder, which opensinto the cylinder; and a scavenger passage, which connects the crankchamber in a crankcase and said scavenging port, and supplies thefuel-air mixture in the crank chamber to said scavenging port; whereinsaid crankcase is configured in such a way that the front and rearportions, which are separated by a block at a right angle to thecrankshaft which entails the axis of the cylinder, are fixed to eachother by mounting hardware, a scavenger passage is provided inside bothsaid front and rear portions of said crankcase, and the cylinder, whosescavenger passage connects to said scavenger passage in said crankcase,is fixed by mounting hardware to the mounting surface on the top of saidcrankcase in such a way that said scavenger passage runs through themounting surface.
 21. A two-stroke cycle engine according to claim 20,further comprising an air passage which supplies air from an air cleanerto said scavenger passage is formed inside the cylinder, and connects tothe middle portion of said scavenger passage inside the cylinder.
 22. Atwo-stroke cycle engine according to claim 20, wherein a pair ofscavenging ports are provided along the circumference of the cylinder, apair of scavenger passages runs from the outlets in the crank chamber tothe scavenging ports, said pair of scavenger passages run through theblock separating the halves of the crankcase, and they should bearranged symmetrically along the front-to-rear dimension of the engine.23. A two-stroke cycle engine, comprising: a scavenger passage whichconnects the crankcase and the scavenging port on the side of thecylinder; an air passage connected to the midpoint of the scavengerpassage, which supplies scavenging air from an air cleaner to thescavenger passage; and a fuel passage, which supplies the fuel-airmixture produced in the carburetor to the crankcase; wherein said aircleaner has two air passages running from it in parallel, the first oneis connected to said air passage, and the second one is connected to theair inlet of the carburetor to provide air for said fuel passage, and achoke valve on the air cleaner is provided to open and close both ofsaid first and second air passages.
 24. A two-stroke cycle engineaccording to claim 23, wherein said choke valve comprises a rotary valvewhich, when rotated, opens or closes the inlets of said first and secondair passages, and a knob by which said valve can be rotated.
 25. Atwo-stroke cycle engine according to claim 24, wherein a choke of saidchoke valve engages with the case of said air cleaner in such a way thatit is free to rotate, the flat surface of said choke comprises a sheetwhich covers or uncovers the inlets of said first and second airpassages, a sealing ring consisting of an elastic material presses theflat surface of said choke against the openings of the inlets by elasticforce and forms a fluid seal around the valve shaft with respect to thecase, a tapered protrusion is formed on the flat surface of the case ofsaid air cleaner, when the rotary knob of said choke valve strikes theprotrusion, then the flat surface of the valve is pressed against theopening of either the first or the second of the two air passages.
 26. Atwo-stroke cycle engine, comprising: a rotary valve installed on thecase in such a way that it is free to rotate which, when rotated, opensand closes the two air passages; and a rotary knob which operates thevalve; wherein the front surface of said rotary valve comprises a sheetwhich covers or uncovers the inlets of the two air passages, a sealingring consisting of an elastic material presses the flat surface of thevalve against the openings of the inlets by elastic force and forms afluid seal around the valve shaft with respect to the interior of thecase.
 27. A two-stroke cycle engine with a scavenger passage whichconnects the crankcase and the scavenging port on the side of thecylinder, which opens into the cylinder and supplies the fuel-airmixture in the crankcase to the scavenging port, wherein said scavengerpassages run in both the crankcase and the cylinder, the front and rearportions of the crankcase, separated by a block at a right angle to thecrankshaft, which entails the axis of the cylinder, are fixed to eachother at the block surface by mounting hardware to form a unitarycrankcase, and the cylinder, whose scavenger passage connects to that inthe crankcase, is fixed by mounting hardware to the mounting surface atthe top of the crankcase.